Graphics subsystem with coarse subpixel correction

ABSTRACT

A graphics processing system in which sub-pixel correction is implemented in a new and more economical way. A half-pixel offset is originally imposed on both the X and Y axes, so the sub-pixel correction value may be positive or negative. A relatively coarse resolution is used for subpixel correction, so that the DDAs (and other functional blocks) do not have to perform a full multiply: instead they merely perform simple add operations (addition of partial products) to derive the necessary offset from the delta-X values, using a proportionality constant provided by the rasterizer. The DDAs preferably include a &#34;guard band&#34; in their calculations, so that values which exceed the maximum (e.g. 255, for 8 bits of subpixel resolution) do not wrap to zero; instead the output is held at the maximum until the computed value comes down below the maximum.

BACKGROUND AND SUMMARY OF THE INVENTION

The present application relates to computer graphics and animation systems, and particularly to graphics rendering hardware.

Computer Graphics and Rendering

Modern computer systems normally manipulate graphical objects as high-level entities. For example, a solid body may be described as a collection of triangles with specified vertices, or a straight line segment may be described by listing its two endpoints with three-dimensional or two-dimensional coordinates. Such high-level descriptions are a necessary basis for high-level geometric manipulations, and also have the advantage of providing a compact format which does not consume memory space unnecessarily.

Such higher-level representations are very convenient for performing the many required computations. For example, ray-tracing or other lighting calculations may be performed, and a projective transformation can be used to reduce a three-dimensional scene to its two-dimensional appearance from a given viewpoint. However, when an image containing graphical objects is to be displayed, a very low-level description is needed. For example, in a conventional CRT display, a "flying spot" is moved across the screen (one line at a time), and the beam from each of three electron guns is switched to a desired level of intensity as the flying spot passes each pixel location. Thus at some point the image model must be translated into a data set which can be used by a conventional display. This operation is known as "rendering."

The graphics-processing system typically interfaces to the display controller through a "frame store" or "frame buffer" of special two-port memory, which can be written to randomly by the graphics processing system, but also provides the synchronous data output needed by the video output driver. (Digital-to-analog conversion is also provided after the frame buffer.) Such a frame buffer is usually implemented using VRAM memory chips (or sometimes with DRAM and special DRAM controllers). This interface relieves the graphics-processing system of most of the burden of synchronization for video output. Nevertheless, the amounts of data which must be moved around are very sizable, and the computational and data-transfer burden of placing the correct data into the frame buffer can still be very large.

Even if the computational operations required are quite simple, they must be performed repeatedly on a large number of datapoints. For example, in a typical 1995 high-end configuration, a display of 1280×1024 elements may need to be refreshed at 72 Hz, with a color resolution of 24 bits per pixel. If blending is desired, additional bits (e.g. another 8 bits per pixel) will be required to store an "alpha" or transparency value for each pixel. This implies manipulation of more than 3 billion bits per second, without allowing for any of the actual computations being performed. Thus it may be seen that this is an environment with unique data manipulation requirements.

If the display is unchanging, no demand is placed on the rendering operations. However, some common operations (such as zooming or rotation) will require every object in the image space to be re-rendered. Slow rendering will make the rotation or zoom appear jerky. This is highly undesirable. Thus efficient rendering is an essential step in translating an image representation into the correct pixel values. This is particularly true in animation applications, where newly rendered updates to a computer graphics display must be generated at regular intervals.

The rendering requirements of three-dimensional graphics are particularly heavy. One reason for this is that, even after the three-dimensional model has been translated to a two-dimensional model, some computational tasks may be bequeathed to the rendering process. (For example, color values will need to be interpolated across a triangle or other primitive.) These computational tasks tend to burden the rendering process. Another reason is that since three-dimensional graphics are much more lifelike, users are more likely to demand a fully rendered image. (By contrast, in the two-dimensional images created e.g. by a GUI or simple game, users will learn not to expect all areas of the scene to be active or filled with information.)

FIG. 1A is a very high-level view of other processes performed in a 3D graphics computer system. A three dimensional image which is defined in some fixed 3D coordinate system (a "world" coordinate system) is transformed into a viewing volume (determined by a view position and direction), and the parts of the image which fall outside the viewing volume are discarded. The visible portion of the image volume is then projected onto a viewing plane, in accordance with the familiar rules of perspective. This produces a two-dimensional image, which is now mapped into device coordinates. It is important to understand that all of these operations occur prior to the operations performed by the rendering subsystem of the present invention. FIG. 1B is an expanded version of FIG. 1A, and shows the flow of operations defined by the OpenGL standard.

A vast amount of engineering effort has been invested in computer graphics systems, and this area is one of increasing activity and demands. Numerous books have discussed the requirements of this area; see, e.g., ADVANCES IN COMPUTER GRAPHICS (ed. Enderle 1990-); Chellappa and Sawchuk, DIGITAL IMAGE PROCESSING AND ANALYSIS (1985); COMPUTER GRAPHICS HARDWARE (ed. Reghbati and Lee 1988); COMPUTER GRAPHICS: IMAGE SYNTHESIS (ed. Joy et al.); Foley et at., FUNDAMENTALS OF INTERACTIVE COMPUTER GRAPHICS (2.ed. 1984); Foley, COMPUTER GRAPHICS PRINCIPLES & PRACTICE (2.ed. 1990); Foley, INTRODUCTION TO COMPUTER GRAPHICS (1994); Giloi, Interactive Computer Graphics (1978); Hearn and Baker, COMPUTER GRAPHICS (2.ed. 1994); Hill, COMPUTER GRAPHICS (1990); Latham, DICTIONARY OF COMPUTER GRAPHICS (1991); Magnenat-Thalma, IMAGE SYNTHESIS THEORY & PRACTICE (1988); Newman and Sproull, PRINCIPLES OF INTERACTIVE COMPUTER GRAPHICS (2.ed. 1979); PICTURE ENGINEERING (ed. Fu and Kunii 1982); PICTURE PROCESSING & DIGITAL FILTERING (2.ed. Huang 1979); Prosise, How COMPUTER GRAPHICS WORK (1994); Rimmer, BIT MAPPED GRAPHICS (2.ed. 1993); Salmon, COMPUTER GRAPHICS SYSTEMS & CONCEPTS (1987); Schachter, COMPUTER IMAGE GENERATION (1990); Watt, THREE-DIMENSIONAL COMPUTER GRAPHICS (2.ed. 1994); Scott Whitman, MULTIPROCESSOR METHODS FOR COMPUTER GRAPHICS RENDERING; the SIGGRAPH PROCEEDINGS for the years 1980-1994; and the IEEE Computer Graphics and Applications magazine for the years 1990-1994; all of which are hereby incorporated by reference.

Background: Use of Subpixel Data

One option in rendering is to perform calculations with more bits of precision than are required for the maximum number of addressable pixels. This permits the system to recognize the difference between a pixel which is 90% covered (by the object being rendered) and a pixel which is 10% covered. This extra information (referred to as "subpixel" data) can be used in a variety of ways; one familiar example is antialiasing, in which the apparent jaggedness of a line is reduced by filling in the edge pixels with a gray-scale value which is proportional to the fraction of the pixel covered.

However, there are other uses for subpixel information. A key use (and one to which the disclosed inventions are particularly directed) is to get higher quality pictures by sampling the continuous functions representing the color, depth, etc. gradients over the area of the triangle at a consistent sample location (i.e. the center of the pixel). The input parameters (i.e. start values at vertices) do not necessarily coincide with the center of the pixel, so unless these are adjusted the sample point has been effectively offset from the desired position. In many cases this offset from the true position will not make any visible difference, but when the gradients are extreme a small offset can cause the sampled parameter to overflow or underflow the permitted range of values. This may result, for example, in an abrupt transition from maximum to minimum values. This is inevitably very noticeable, and is known as the "bright edge" artefact.

Apart from "bright edge" artifacts, many color errors are not very noticeable, since the human eye is not very sensitive to subtle errors in the exact color shade. However, other parameters may be more critical: for example, a slight error in the depth calculations can result in two polygons being drawn in the wrong order, which can be very obvious. This error usually happens along a common edge, in which case a very noticeable castellation effect may occur, in which alternate groups of pixels along the edge take their colors from different polygons.

Calculations with subpixel resolution are useful in obtaining the highest possible quality, but they are computationally burdensome.

The scan conversion process for a triangle when interpolating a parameter (i.e. depth, red, green, etc.) normally includes the steps of 1) stepping along a leading edge, and 2) incrementing the parameters. The parameter values at each line along the leading edge are then used as the start value to iterate along a span. For this process to give accurate results, the initial value of the parameters for a scan line need to modified to take into account the fact that the sample point on the edge is not usually coincident with the pixels center where the sample point is required. (In the presently preferred embodiment, the dominant edge of a primitive (if there is one) is normally used as the leading edge, and hence the leading edge will henceforth be referred to as the dominant edge (see FIG. 4A). However, it should be specifically noted that the invention of the present application is also applicable to graphics systems where the leading edge is necessarily the dominant edge.)

FIG. 5 illustrates this, as described in detail below.

The parameter value needs to be modified by an amount PROPORTIONAL to the distance between the pixel's center and the mathematically drawn leading edge. To find this proportional amount requires a multiply operation, for each parameter, for each new scan line.

Traditionally such calculations have required the addition of multipliers to the DDA blocks. DDAs are relatively simple addition-based logic block, which do not require multiplication, so the inclusion of multiply operations in the DDAs is a major burden on the area efficiency of a design implementation.

Background: Parallelism in Graphics Processing

Due to the large number of at least partially independent operations which are performed in rendering, many proposals have been made to use some form of parallel architecture for graphics (and particularly for rendering). See, for example, the special issue of Computer Graphics on parallel rendering (September 1994). Other approaches may be found in earlier patent filings by the assignee of the present application and its predecessors, e.g. U.S. Pat. No. 5,195,186, and published PCT applications PCT/GB90/00987, PCT/GB90/01209, PCT/GB90/01210, PCT/GB90/01212, PCT/GB90/01213, PCT/GB90/01214, PCT/GB90/01215, and PCT/GB90/01216, all of which are hereby incorporated by reference.

Background: Pipelined Processing Generally

There are several general approaches to parallel processing. One of the basic approaches to achieving parallelism in computer processing is a technique known as pipelining. In this technique the individual processors are, in effect, connected in series in an assembly-line configuration: one processor performs a first set of operations on one chunk of data, and then passes that chunk along to another processor which performs a second set of operations, while at the same time the first processor performs the first set operations again on another chunk of data. Such architectures are generally discussed in Kogge, THE ARCHITECTURE OF PIPELINED COMPUTERS (1981), which is hereby incorporated by reference.

Background: The OpenGL™ Standard

The "OpenGL" standard is a very important software standard for graphics applications. In any computer system which supports this standard, the operating system(s) and application software programs can make calls according to the OpenGL standards, without knowing exactly what the hardware configuration of the system is.

The OpenGL standard provides a complete library of low-level graphics manipulation commands, which can be used to implement three-dimensional graphics operations. This standard was originally based on the proprietary standards of Silicon Graphics, Inc., but was later transformed into an open standard. It is now becoming extremely important, not only in high-end graphics-intensive workstations, but also in high-end PCs. OpenGL is supported by Windows NT™, which makes it accessible to many PC applications.

The OpenGL specification provides some constraints on the sequence of operations. For instance, the color DDA operations must be performed before the texturing operations, which must be performed before the alpha operations. (A "DDA" or digital differential analyzer, is a conventional piece of hardware used to produce linear gradation of color (or other) values over an image area.)

Other graphics interfaces (or "APIs"), such as PHIGS or XGL, are also current as of 1995; but at the lowest level, OpenGL is a superset of most of these.

The OpenGL standard is described in the OPENGL PROGRAMMING GUIDE (1993), the OPENGL REFERENCE MANUAL (1993), and a book by Segal and Akeley (of SGI) entitled THE OPENGL GRAPHICS SYSTEM: A SPECIFICATION (Version 1.0), all of which are hereby incorporated by reference.

FIG. 1B is an expanded version of FIG. 1A, and shows the flow of operations defined by the OpenGL standard. Note that the most basic model is carried in terms of vertices, and these vertices are then assembled into primitives (such as triangles, lines, etc.). After all manipulation of the primitives has been completed, the rendering operations will translate each primitive into a set of "fragments." (A fragment is the portion of a primitive which affects a single pixel.) Again, it should be noted that all operations above the block marked "Rasterization" would be performed by a host processor, or possibly by a "geometry engine" (i.e. a dedicated processor which performs rapid matrix multiplies and related data manipulations), but would normally not be performed by a dedicated rendering processor such as that of the presently preferred embodiment.

Innovative System and Preferred System Context

The present invention provides a new approach to these needs. In this approach calculations are performed with subpixel resolution, without requiring any multiplications in the various DDA units.

This is achieved by using a relatively coarse resolution for subpixel correction, so that the DDAs (and other functional blocks) do not have to perform a full multiply: instead they merely perform simple add operations (addition of partial products) to provide the necessary offset. The DDAs preferably include a "guard band" in their calculation logic, so that values which exceed the maximum parameter value (e.g. 255, for 8 bits of resolution) do not wrap to zero; instead the output is held at the maximum until the computed value comes down below the maximum. Preferably a half-pixel offset is used on both the X and Y axes, so the subpixel correction value may be positive or negative.

The added cost of providing for this technique is very small: just the addition of a multiplexer and some simple sequencing logic in each DDA. To achieve this, the rasterizer needs to provide a measure of the distance between the pixel's center and the edge. This value does not need to have a particularly high accuracy (1/16 pixel has been found to be good enough) to give good visual results.

This idea can be implemented in other parallel rendering architectures, and is not limited to the message-passing architecture used in the presently preferred embodiment.

Some notable features and advantages of this invention, in various embodiments, include the following:

1. No new parameters are needed as the same dx value as used during the X fill is used and the 1/2, 1/4, 1/8 etc values are just picked out from the dx value by the multiplexers.

2. No new arithmetic elements are needed as the adder (and saturation logic) already exist, together with the registers and data paths to accumulate the results.

3. Reduced resolution can be used with acceptable results, so a serial method of summing in the partial products can be used.

4. The burden on the rasterizer is very little in providing the error term.

5. Each DDA does the correction in parallel, so the cost is fixed per scan line independent of the number of parameters to interpolate. When 4 subpixel bits are used, the cost is on average 2 cycles (worst case 4 cycles), so the overhead is very little and often hidden under other scan line costs such as a page break in memory.

A further innovative point is the use of a DDA unit with a guard band in its calculations. (That is, the DDA, in the presently preferred embodiment, provides a parameter output with N bits of resolution, but internally computes the parameter value with one or more MSBs added onto the N.) This prevents visible artifacts from occurring when an overflow or underflow occurs in computation.

BRIEF DESCRIPTION OF THE DRAWING

The disclosed inventions will be described with reference to the accompanying drawings, which show important sample embodiments of the invention and which are incorporated in the specification hereof by reference, wherein:

FIG. 1A described above, is an overview of key elements and processes in a 3D graphics computer system.

FIG. 1B is an expanded version of FIG. 1A, and shows the flow of operations defined by the OpenGL standard.

FIG. 2A is an overview of the graphics rendering chip of the presently preferred embodiment.

FIG. 2B is an alternative embodiment of the graphics rendering chip of FIG. 2A, which includes additional texture-manipulation capabilities.

FIG. 2C is a more schematic view of the sequence of operations performed in the graphics rendering chip of FIG. 2A.

FIG. 3A shows a sample graphics board which incorporates the chip of FIG. 2A.

FIG. 3B shows another sample graphics board implementation, which differs from the board of FIG. 3A in that more memory and an additional component is used to achieve higher performance.

FIG. 3C shows another graphics board, in which the chip of FIG. 2A shares access to a common frame store with GUI accelerator chip.

FIG. 3D shows another graphics board, in which the chip of FIG. 2A shares access to a common frame store with a video coprocessor (which may be used for video capture and playback functions.

FIG. 4A illustrates the definition of the dominant side and the subordinate sides of a triangle.

FIG. 4B illustrates the sequence of rendering an Antialiased Line primitive.

FIG. 5 illustrates the use of subpixel bits of resolution in rendering.

FIG. 6 shows a block diagram of a color component DDA.

FIG. 7 schematically shows the guard band provided by the internal computation performed in the DDA.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The numerous innovative teachings of the present application will be described with particular reference to the presently preferred embodiment (by way of example, and not of limitation). The presently preferred embodiment is a GLINT™300SX™3D rendering chip. The Hardware Reference Manual and Programmer's Reference Manual for this chip describe further details of this sample embodiment, and are both hereby incorporated by reference. Both are available, as of the effective filing date of this application, from 3Dlabs Inc. Ltd., 2010 N. 1st St., suite 403, San Jose Calif. 95131.

Definitions

The following definitions may help in understanding the exact meaning of terms used in the text of this application:

application: a computer program which uses graphics animation.

depth (Z) buffer: A memory buffer containing the depth component of a pixel. Used to, for example, eliminate hidden surfaces.

blt double-buffering: A technique for achieving smooth animation, by rendering only to an undisplayed back buffer, and then copying the back buffer to the front once drawing is complete.

FrameCount Planes: Used to allow higher animation rates by enabling DRAM local buffer pixel data, such as depth (Z), to be cleared down quickly.

frame buffer: An area of memory containing the displayable color buffers (front, back, left, right, overlay, underlay). This memory is typically separate from the local buffer.

local buffer: An area of memory which may be used to store non-displayable pixel information: depth(Z), stencil, FrameCount and GID planes. This memory is typically separate from the framebuffer.

pixel: Picture element. A pixel comprises the bits in all the buffers (whether stored in the local buffer or framebuffer), corresponding to a particular location in the framebuffer.

stencil buffer: A buffer used to store information about a pixel which controls how subsequent stencilled pixels at the same location may be combined with the current value in the framebuffer. Typically used to mask complex two-dimensional shapes.

Preferred Chip Embodiment--Overview

The GLINT™ high performance graphics processors combine workstation class 3D graphics acceleration, and state-of-the-art 2D performance in a single chip. All 3D rendering operations are accelerated by GLINT, including Gouraud shading, texture mapping, depth buffering, anti-aliasing, and alpha blending.

The scalable memory architecture of GLINT makes it ideal for a wide range of graphics products, from PC boards to high-end workstation accelerators.

There will be several of the GLINT family of graphics processors: the GLINT 300SX™ is the primary preferred embodiment which is described herein in great detail, and the GLINT 300TX™ is a planned alternative embodiment which is also mentioned hereinbelow. The two devices are generally compatible, with the 300TX adding local texture storage and texel address generation for all texture modes.

FIG. 2A is an overview of the graphics rendering chip of the presently preferred embodiment (i.e. the GLINT 300SX™).

General Concept

The overall architecture of the GLINT chip is best viewed using the software paradigm of a message passing system. In this system all the processing blocks are connected in a long pipeline with communication with the adjacent blocks being done through message passing. Between each block there is a small amount of buffering, the size being specific to the local communications requirements and speed of the two blocks.

The message rate is variable and depends on the rendering mode. The messages do not propagate through the system at a fixed rate typical of a more traditional pipeline system. If the receiving block can not accept a message, because its input buffer is full, then the sending block stalls until space is available.

The message structure is fundamental to the whole system as the messages are used to control, synchronize and inform each block about the processing it is to undertake. Each message has two fields--a 32 bit data field and a 9 bit tag field. (This is the minimum width guaranteed, but some local block to block connections may be wider to accommodate more data.) The data field will hold color information, coordinate information, local state information, etc. The tag field is used by each block to identify the message type so it knows how to act on it.

Each block, on receiving a message, can do one of several things:

•Not recognize the message so it just passes it on to the next block.

•Recognize it as updating some local state (to the block) so the local state is updated and the message terminated, i.e. not passed on to the next block.

•Recognize it as a processing action, and if appropriate to the unit, the processing work specific to the unit is done. This may entail sending out new messages such as Color and/or modifying the initial message before sending it on. Any new messages are injected into the message stream before the initial message is forwarded on. Some examples will clarify this.

When the Depth Block receives a message `new fragment`, it will calculate the corresponding depth and do the depth test. If the test passes then the `new fragment` message is passed to the next unit. If the test fails then the message is modified and passed on. The temptation is not to pass the message on when the test fails (because the pixel is not going to be updated), but other units downstream need to keep their local DDA units in step.

(In the present application, the messages are being described in general terms so as not to be bogged down in detail at this stage. The details of what a `new fragment` message actually specifies (i.e. coordinate, color information) is left till later. In general, the term "pixel" is used to describe the picture element on the screen or in memory. The term "fragment" is used to describe the part of a polygon or other primitive which projects onto a pixel. Note that a fragment may only cover a part of a pixel.)

When the Texture Read Unit (if enabled) gets a `new fragment` message, it will calculate the texture map addresses, and will accordingly provide 1, 2, 4 or 8 texels to the next unit together with the appropriate number of interpolation coefficients.

Each unit and the message passing are conceptually running asynchronous to all the others. However, in the presently preferred embodiment there is considerable synchrony because of the common clock.

How does the host process send messages? The message data field is the 32 bit data written by the host, and the message tag is the bottom 9 bits of the address (excluding the byte resolution address lines). Writing to a specific address causes the message type associated with that address to be inserted into the message queue. Alternatively, the on-chip DMA controller may fetch the messages from the host's memory.

The message throughput, in the presently preferred embodiment, is 50M messages per second and this gives a fragment throughput of up to 50M per second, depending on what is being rendered. Of course, this rate will predictably be further increased over time, with advances in process technology and clock rates.

Linkage

The block diagram of FIG. 2A shows how the units are connected together in the GLINT 300SX embodiment, and the block diagram of FIG. 2B shows how the units are connected together in the GLINT 300TX embodiment. Some general points are:

•The following functionality is present in the 300TX, but missing from the 300SX: The Texture Address (TAddr) and Texture Read (TRd) Units are missing. Also, the router and multiplexer are missing from this section, so the unit ordering is Scissor/Stipple, Color DDA, Texture Fog Color, Alpha Test, LB Rd, etc.

•In the embodiment of FIG. 2B, the order of the units can be configured in two ways. The most general order (Router, Color DDA, Texture Unit, Alpha Test, LB Rd, GID/Z/Stencil, LB Wr, Multiplexer) and will work in all modes of OpenGL. However, when the alpha test is disabled it is much better to do the Graphics ID, depth and stencil tests before the texture operations rather than after. This is because the texture operations have a high processing cost and this should not be spent on fragments which are later rejected because of window, depth or stencil tests.

•The loop back to the host at the bottom provides a simple synchronization mechanism. The host can insert a Sync command and when all the preceding rendering has finished the sync command will reach the bottom host interface which will notify the host the sync event has occurred.

Benefits

The very modular nature of this architecture gives great benefits. Each unit lives in isolation from all the others and has a very well defined set of input and output messages. This allows the internal structure of a unit (or group of units) to be changed to make algorithmic/speed/gate count trade-offs.

The isolation and well defined logical and behavioral interface to each unit allows much better testing and verification of the correctness of a unit.

The message passing paradigm is easy to simulate with software, and the hardware design is nicely partitioned. The architecture is self synchronizing for mode or primitive changes.

The host can mimic any block in the chain by inserting messages which that block would normally generate. These message would pass through the earlier blocks to the mimicked block unchanged and from then onwards to the rest of the blocks which cannot tell the message did not originate from the expected block. This allows for an easy work around mechanism to correct any flaws in the chip. It also allows other rasterization paradigms to be implemented outside of the chip, while still using the chip for the low level pixel operations.

"A Day in the Life of a Triangle"

Before we get too detailed in what each unit does it is worth while looking in general terms at how a primitive (e.g. triangle) passes through the pipeline, what messages are generated, and what happens in each unit. Some simplifications have been made in the description to avoid detail which would otherwise complicate what is really a very simple process. The primitive we are going to look at is the familiar Gouraud shaded Z buffered triangle, with dithering. It is assumed any other state (i.e. depth compare mode) has been set up, but (for simplicity) such other states will be mentioned as they become relevant.

•The application generates the triangle vertex information and makes the necessary OpenGL calls to draw it.

•The OpenGL server/library gets the vertex information, transforms, clips and lights it. It calculates the initial values and derivatives for the values to interpolate (X_(left), X_(right), red, green, blue and depth) for unit change in dx and dxdy_(left). All these values are in fixed point integer and have unique message tags. Some of the values (the depth derivatives) have more than 32 bits to cope with the dynamic range and resolution so are sent in two halves Finally, once the derivatives, start and end values have been sent to GLINT the `render triangle` message is sent.

•On GLINT: The derivative, start and end parameter messages are received and filter down the message stream to the appropriate blocks. The depth parameters and derivatives to the Depth Unit; the RGB parameters and derivative to the Color DDA Unit; the edge values and derivatives to the Rasterizer Unit.

•The `render triangle` message is received by the rasterizer unit and all subsequent messages (from the host) are blocked until the triangle has been rasterized (but not necessarily written to the frame store). A `prepare to render` message is passed on so any other blocks can prepare themselves.

•The Rasterizer Unit walks the left and right edges of the triangle and fills in the spans between. As the walk progresses messages are send to indicate the direction of the next step: StepX or StepYDomEdge. The data field holds the current (x, y) coordinate. One message is sent per pixel within the triangle boundary. The step messages are duplicated into two groups: an active group and a passive group. The messages always start off in the active group but may be changed to the passive group if this pixel fails one of the tests (e.g. depth) on its path down the message stream. The two groups are distinguished by a single bit in the message tag. The step messages (in either form) are always passed throughout the length of the message stream, and are used by all the DDA units to keep their interpolation values in-step. The step message effectively identifies the fragment and any other messages pertaining to this fragment will always precede the step message in the message stream.

•The Scissor and Stipple Unit. This unit does 4 tests on the fragment (as embodied by the active step message). The screen scissor test takes the coordinates associated with the step message, converts them to be screen relative (if necessary) and compares them against the screen boundaries. The other three tests (user scissor, line stipple and area stipple) are disabled for this example. If the enabled tests pass then the active step is forwarded onto the next unit, otherwise it is changed into a passive step and then forwarded.

•The Color DDA unit responds to an active step message by generating a Color message and sending this onto the next unit. The active step message is then forwarded to the next unit. The Color message holds, in the data field, the current RGBA value from the DDA. If the step message is passive then no Color message is generated. After the Color message is sent (or would have been sent) the step message is acted on to increment the DDA in the correct direction, ready for the next pixel.

•Texturing, Fog and Alpha Tests Units are disabled so the messages just pass through these blocks.

•In general terms the Local Buffer Read Unit reads the Graphic ID, Stencil and Depth information from the Local Buffer and passes it onto the next unit. More specifically it does:

1. If the step message is passive then no further action occurs.

2. On an active step message it calculates the linear address in the local buffer of the required data. This is done using the (X, Y) position recorded in the step message and locally stored information on the `screen width` and window base address. Separate read and write addresses are calculated.

3. The addresses are passed to the Local Buffer Interface Unit and the identified local buffer location read. The write address is held for use later.

4. Sometime later the local buffer data is returned and is formatted into a consistent internal format and inserted into a `Local Buffer Data` message and passed on to the next unit. The message data field is made wider to accommodate the maximum Local Buffer width of 52 bits (32 depth, 8 stencil, 4 graphic ID, 8 frame count) and this extra width just extends to the Local Buffer Write block. The actual data read from the local buffer can be in several formats to allow narrower width memories to be used in cost sensitive systems. The narrower data is formatted into a consistent internal format in this block.

•The Graphic ID, Stencil and Depth Unit just passes the Color message through and stores the LBData message until the step message arrives. A passive step message would just pass straight through. When the active step message is received the internal Graphic ID, stencil and depth values are compared with the ones in the LBData message as specified by this unit's mode information. If the enabled tests pass then the new local buffer data is sent in the LBWriteData message to the next unit and the active step message forwarded. If any of the enabled tests fail then an LBCancelWrite message is sent followed by the equivalent passive step message. The depth DDA is stepped to update the local depth value.

•The Local Buffer Write Unit performs any writes which are necessary. The LBWriteData message has its data formatted into the external local buffer format and this is posted to the Local Buffer Interface Unit to be written into the memory (the write address is already waiting in the Local Buffer Interface Unit). The LBWriteCancel message just informs the Local Buffer Interface Unit that the pending write address is no longer needed and can be discarded. The step message is just passed through.

•In general terms the Framebuffer Read Unit reads the color information from the framebuffer and passes it onto the next unit. More specifically it does:

1. If the step message is passive then no further action occurs.

2. On an active step message it calculates the linear address in the framebuffer of the required data. This is done using the (X, Y) position recorded in the step message and locally stored information on the `screen width` and window base address. Separate read and write addresses are calculated.

3. The addresses are passed to the Framebuffer Interface Unit and the identified framebuffer location read. The write address is held for use later.

4. Sometime later the color data is returned and inserted into a `Frame Buffer Data` message and passed on to the next unit. The actual data read from the framestore can be in several formats to allow narrower width memories to be used in cost sensitive systems. The formatting of the data is deferred until the Alpha Blend Unit as it is the only unit which needs to match it up with the internal formats. In this example no alpha blending or logical operations are taking place, so reads are disabled and hence no read address is sent to the Framebuffer Interface Unit. The Color and step messages just pass through.

•The Alpha Blend Unit is disabled so just passes the messages through.

•The Dither Unit stores the Color message internally until an active step is received. On receiving this it uses the least significant bits of the (X, Y) coordinate information to dither the contents of the Color message. Part of the dithering process is to convert from the internal color format into the format of the framebuffer. The new color is inserted into the Color message and passed on, followed by the step message.

•The Logical Operations are disabled so the Color message is just converted into the FBWriteData message Oust the tag changes) and forwarded on to the next unit. The step message just passes through.

•The Framebuffer Write Unit performs any writes which are necessary. The FBWriteData message has its data posted to the Framebuffer Interface Unit to be written into the memory (the write address is already waiting in the Framebuffer Interface Unit). The step message is just passed through.

•The Host Out Unit is mainly concerned with synchronization with the host so for this example will just consume any messages which reach this point in the message stream.

This description has concentrated on what happens as one fragment flows down the message stream. It is important to remember that at any instant in time there are many fragments flowing down the message stream and the further down they reach the more processing has occurred.

Interfacing Between Blocks

FIG. 2A shows the FIFO buffering and lookahead connections which are used in the presently preferred embodiment. The FIFOs are used to provide an asynchronous interface between blocks, but are expensive in terms of gate count. Note that most of these FIFOs are only one stage deep (except where indicated), which reduces their area. To maintain performance, lookahead connections are used to accelerate the "startup" of the pipeline. For example, when the Local-Buffer-Read block issues a data request, the Texture/Fog/Color blocks also receive this, and begin to transfer data accordingly. Normally a single-entry deep FIFO cannot be read and written in the same cycle, as the writing side doesn't know that the FIFO is going to be read in that cycle (and hence become eligible to be written). The look-ahead feature give the writing side this insight, so that single-cycle transfer can be achieved. This accelerates the throughput of the pipeline.

Programming Model

The following text describes the programming model for GLINT.

GLINT as a Register file

The simplest way to view the interface to GLINT is as a flat block of memory-mapped registers (i.e. a register file). This register file appears as part of Region 0 of the PCI address map for GLINT. See the GLINT Hardware Reference Manual for details of this address map.

When a GLINT host software driver is initialized it can map the register file into its address space. Each register has an associated address tag, giving its offset from the base of the register file (since all registers reside on a 64-bit boundary, the tag offset is measured in multiples of 8 bytes). The most straightforward way to load a value into a register is to write the data to its mapped address. In reality the chip interface comprises a 16 entry deep FIFO, and each write to a register causes the written value and the register's address tag to be written as a new entry in the FIFO.

Programming GLINT to draw a primitive consists of writing initial values to the appropriate registers followed by a write to a command register. The last write triggers the start of rendering.

GLINT has approximately 500 registers. All registers are 32 bits wide and should be 32-bit addressed. Many registers are split into bit fields, and it should be noted that bit 0 is the least significant bit.

Register Types

GLINT has three main types of register:

•Control Registers

•Command Registers

•Internal Registers

Control Registers are updated only by the host--the chip effectively uses them as read-only registers. Examples of control registers are the Scissor Clip unit min and max registers. Once initialized by the host, the chip only reads these registers to determine the scissor clip extents.

Command Registers are those which, when written to, typically cause the chip to start rendering (some command registers such as ResetPickResult or Sync do not initiate rendering). Normally, the host will initialize the appropriate control registers and then write to a command register to initiate drawing. There are two types of command registers: begin-draw and continue-draw. Begin-draw commands cause rendering to start with those values specified by the control registers. Continue-draw commands cause drawing to continue with internal register values as they were when the previous drawing operation completed. Making use of continue-draw commands can significantly reduce the amount of data that has to be loaded into GLINT when drawing multiple connected objects such as polylines. Examples of command registers include the Render and ContinueNewLine registers.

For convenience this application will usually refer to "sending a Render command to GLINT" rather than saying (more precisely) "the Render Command register is written to, which initiates drawing".

Internal Registers are not accessible to host software. They are used internally by the chip to keep track of changing values. Some control registers have corresponding internal registers. When a begin-draw command is sent and before rendering starts, the internal registers are updated with the values in the corresponding control registers. If a continue-draw command is sent then this update does not happen and drawing continues with the current values in the internal registers. For example, if a line is being drawn then the StartXDom and StartY control registers specify the (x, y) coordinates of the first point in the line. When a begin-draw command is sent these values are copied into internal registers. As the line drawing progresses these internal registers are updated to contain the (x, y) coordinates of the pixel being drawn. When drawing has completed the internal registers contain the (x, y) coordinates of the next point that would have been drawn. If a continue-draw command is now given these final (x, y) internal values are not modified and further drawing uses these values. If a begin-draw command had been used the internal registers would have been reloaded from the StartXDom and StartY registers.

For the most part internal registers can be ignored. It is helpful to appreciate that they exist in order to understand the continue-draw commands.

GLINT I/O Interface

There are a number of ways of loading GLINT registers for a given context:

•The host writes a value to the mapped address of the register

•The host writes address-tag/data pairs into a host memory buffer and uses the on-chip DMA to transfer this data to the FIFO.

•The host can perform a Block Command Transfer by writing address and data values to the FIFO interface registers.

In all cases where the host writes data values directly to the chip (via the register file) it has to worry about FIFO overflow. The InFIFOSpace register indicates how many free entries remain in the FIFO. Before writing to any register the host must ensure that there is enough space left in the FIFO. The values in this register can be read at any time. When using DMA, the DMA controller will automatically ensure that there is room in the FIFO before it performs further transfers. Thus a buffer of any size can be passed to the DMA controller.

FIFO Control

The description above considered the GLINT interface to be a register file. More precisely, when a data value is written to a register this value and the address tag for that register are combined and put into the FIFO as a new entry. The actual register is not updated until GLINT processes this entry. In the case where GLINT is busy performing a time consuming operation (e.g. drawing a large texture mapped polygon), and not draining the FIFO very quickly, it is possible for the FIFO to become full. If a write to a register is performed when the FIFO is full no entry is put into the FIFO and that write is effectively lost.

The input FIFO is 16 entries deep and each entry consists of a tag/data pair. The InFIFOSpace register can be read to determine how many entries are free. The value returned by this register will never be greater than 16.

To check the status of the FIFO before every write is very inefficient, so it is preferably checked before loading the data for each rectangle. Since the FIFO is 16 entries deep, a further optimization is to wait for all 16 entries to be free after every second rectangle. Further optimizations can be made by moving dXDom, dXSub and dY outside the loop (as they are constant for each rectangle) and doing the FIFO wait after every third rectangle.

The InFIFOSpace FIFO control register contains a count of the number of entries currently free in the FIFO. The chip increments this register for each entry it removes from the FIFO and decrements it every time the host puts an entry in the FIFO.

The DMA Interface

Loading registers directly via the FIFO is often an inefficient way to download data to GLINT. Given that the FIFO can accommodate only a small number of entries, GLINT has to be frequently interrogated to determine how much space is left. Also, consider the situation where a given API function requires a large amount of data to be sent to GLINT. If the FIFO is written directly then a return from this function is not possible until almost all the data has been consumed by GLINT. This may take some time depending on the types of primitives being drawn.

To avoid these problems GLINT provides an on-chip DMA controller which can be used to load data from arbitrary sized (<64K 32-bit words) host buffers into the FIFO. In its simplest form the host software has to prepare a host buffer containing register address tag descriptions and data values. It then writes the base address of this buffer to the DMAAddress register and the count of the number of words to transfer to the DMACount register. Writing to the DMACount register starts the DMA transfer and the host can now perform other work. In general, if the complete set of rendering commands required by a given call to a driver function can be loaded into a single DMA buffer then the driver function can return. Meanwhile, in parallel, GLINT is reading data from the host buffer and loading it into its FIFO. FIFO overflow never occurs since the DMA controller automatically waits until there is room in the FIFO before doing any transfers.

The only restriction on the use of DMA control registers is that before attempting to reload the DMACount register the host software must wait until previous DMA has completed. It is valid to load the DMAAddress register while the previous DMA is in progress since the address is latched internally at the start of the DMA transfer.

Using DMA leaves the host free to return to the application, while in parallel, GLINT is performing the DMA and drawing. This can increase performance significantly over loading a FIFO directly. In addition, some algorithms require that data be loaded multiple times (e.g. drawing the same object across multiple clipping rectangles). Since the GLINT DMA only reads the buffer data, it can be downloaded many times simply by restarting the DMA. This can be very beneficial if composing the buffer data is a time consuming task.

The host can use this hardware capability in various ways. For example, a further optional optimization is to use a double buffered mechanism with two DMA buffers. This allows the second buffer to be filled before waiting for the previous DMA to complete, thus further improving the parallelism between host and GLINT processing. Thus, this optimization is dependent on the allocation of the host memory. If there is only one DMA host buffer then either it is being filled or it is being emptied--it cannot be filled and emptied at the same time, since there is no way for the host and DMA to interact once the DMA transfer has started. The host is at liberty to allocate as many DMA buffers as it wants; two is the minimum to do double buffering, but allocating many small buffers is generally better, as it gives the benefits of double buffering together with low latency time, so GLINT is not idle while large buffer is being filled up. However, use of many small buffers is of course more complicated.

In general the DMA buffer format consists of a 32-bit address tag description word followed by one or more data words. The DMA buffer consists of one or more sets of these formats. The following paragraphs describe the different types of tag description words that can be used.

DMA Tag Description Format

There are 3 different tag addressing modes for DMA: hold, increment and indexed. The different DMA modes are provided to reduce the amount of data which needs to be transferred, hence making better use of the available DMA bandwidth. Each of these is described in the following sections.

Hold Format

In this format the 32-bit tag description contains a tag value and a count specifying the number of data words following in the buffer. The DMA controller writes each of the data words to the same address tag. For example, this is useful for image download where pixel data is continuously written to the Color register. The bottom 9 bits specify the register to which the data should be written; the high-order 16 bits specify the number of data words (minus 1) which follow in the buffer and which should be written to the address tag (note that the 2-bit mode field for this format is zero so a given tag value can simply be loaded into the low order 16 bits).

A special case of this format is where the top 16 bits are zero indicating that a single data value follows the tag (i.e. the 32-bit tag description is simply the address tag value itself). This allows simple DMA buffers to be constructed which consist of tag/data pairs.

Increment Format

This format is similar to the hold format except that as each data value is loaded the address tag is incremented (the value in the DMA buffer is not changed; GLINT updates an internal copy). Thus, this mode allows contiguous GLINT registers to be loaded by specifying a single 32-bit tag value followed by a data word for each register. The low-order 9 bits specify the address tag of the first register to be loaded. The 2 bit mode field is set to 1 and the high-order 16 bits are set to the count (minus 1) of the number of registers to update. To enable use of this format, the GLINT register file has been organized so that registers which are frequently loaded together have adjacent address tags. For example, the 32 AreaStipplePattern registers can be loaded as follows:

    ______________________________________                                         AreaStipplePattern0, Count=31, Mode=1                                          row 0 bits                                                                     row 1 bits                                                                     . . .                                                                          row 31 bits                                                                    ______________________________________                                    

Indexed Format

GLINT address tags are 9 bit values. For the purposes of the Indexed DMA Format they are organized into major groups and within each group there are up to 16 tags. The low-order 4 bits of a tag give its offset within the group. The high-order 5 bits give the major group number.

The following Register Table lists the individual registers with their Major Group and Offset in the presently preferred embodiment:

Register Table

The following table lists registers by group, giving their tag values and indicating their type. The register groups may be used to improve data transfer rates to GLINT when using DMA.

The following types of register are distinguished:

    ______________________________________                                                                 Major   Off-                                                                   Group   set                                            Unit     Register       (hex)   (hex)                                                                               Type                                      ______________________________________                                         Rasterizer                                                                              StartXDom      00      0    Control                                            dXDom          00      1    Control                                            StartXSub      00      2    Control                                            dXSub          00      3    Control                                            StartY         00      4    Control                                            dY             00      5    Control                                            Count          00      6    Control                                            Render         00      7    Command                                            ContinueNewLine                                                                               00      8    Command                                            ContinueNewDom 00      9    Command                                            ContinueNewSub 00      A    Command                                            Continue       00      B    Command                                            FlushSpan      00      C    Command                                            BitMaskPattern 00      D    Mixed                                     Rasterizer                                                                              PointTable[0-3]                                                                               01      0-3  Control                                            RasterizerMode 01      4    Control                                   Scissor  ScissorMode    03      3    Control                                   Stipple                                                                                 ScissorMinXY   03      1    Control                                            ScissorMaxXY   03      2    Control                                            ScreenSize     03      3    Control                                            AreaStippleMode                                                                               03      4    Control                                            LineStippleMode                                                                               03      5    Control                                            LoadLineStippleC                                                                              03      6    Control                                            ounters                                                                        UpdateLineStipple                                                                             03      7    Command                                            Counters                                                                       SaveLineStippleSt                                                                             03      8    Command                                            ate                                                                            WindowOrigin   03      9    Control                                   Scissor  AreaStipplePatter                                                                             04      0-F  Control                                   Stipple  n[0-31]        05      0-F                                            Texture  Texel0         0C      0    Control                                   Color/Fog                                                                               Texel1         0C      1    Control                                            Texel2         0C      2    Control                                            Texel3         0C      3    Control                                            Texel4         0C      4    Control                                            Texel5         0C      5    Control                                            Texel6         0C      6    Control                                            Texel7         0C      7    Control                                            Interp0        0C      8    Control                                            Interp1        0C      9    Control                                            Interp2        0C      A    Control                                            Interp3        0C      B    Control                                            Interp4        0C      C    Control                                            TextureFilter  0C      D    Control                                   Texture/Fog                                                                             TextureColorMode                                                                              0D      0    Control                                   Color                                                                                   TextureEnvColor                                                                               0D      1    Control                                            FogMode        0D      2    Control                                            FogColor       0D      3    Control                                            FStart         0D      4    Control                                            dFdx           0D      5    Control                                            dFdyDom        0D      6    Control                                   Color DDA                                                                               RStart         0F      0    Control                                            dRdx           0F      1    Control                                            dRdyDom        0F      2    Control                                            GStart         0F      3    Control                                            dGdx           0F      4    Control                                            dGdyDom        0F      5    Control                                            BStart         0F      6    Control                                            dBdx           0F      7    Control                                            dBdyDom        0F      8    Control                                            AStart         0F      9    Control                                            dAdx           0F      A    Control                                            dAdyDom        0F      B    Control                                            ColorDDAMode   0F      C    Control                                            ConstantColor  0F      D    Control                                            Color          0F      E    Mixed                                     Alpha Test                                                                              AlphaTestMode  10      0    Control                                            AntialiasMode  10      1    Control                                   Alpha Blend                                                                             AlphaBlendMode 10      2    Control                                   Dither   DitherMode     10      3    Control                                   Logical Ops                                                                             FBSoftwareWrite                                                                               10      4    Control                                            Mask                                                                           LogicalOpMode  10      5    Control                                            FBWriteData    10      6    Control                                   LB Read  LBReadMode     11      0    Control                                            LBReadFormat   11      1    Control                                            LBSourceOffset 11      2    Control                                            LBStencil      11      5    Control                                            LBDepth        11      6    Output                                             LBWindowBase   11      7    Control                                   LB Write LBWriteMode    11      8    Control                                            LBWriteFormat  11      9    Control                                   GID/Stencil/                                                                            Window         13      0    Control                                   Depth                                                                                   StencilMode    13      1    Control                                            StencilData    13      2    Control                                            Stencil        13      3    Mixed                                              DepthMode      13      4    Control                                            Depth          13      5    Mixed                                              ZStartU        13      6    Control                                            ZStartL        13      7    Control                                            dZdxU          13      8    Control                                            dZdxL          13      9    Control                                            dZdyDomU       13      A    Control                                            dZdyDomL       13      B    Control                                            FastClearDepth 13      C    Control                                   FB Read  FBReadMode     15      0    Control                                            FBSourceOffset 15      1    Control                                            FBPixelOffset  15      2    Control                                            FBColor        15      3    Output                                             FBWindowBase   15      6    Control                                   FB Write FBWriteMode    15      7    Control                                            FBHardwareWrite                                                                               15      8    Control                                            Mask                                                                           FBBlockColor   15      9    Control                                   Host Out FilterMode     18      0    Control                                            StatisticMode  18      1    Control                                            MinRegion      18      2    Control                                            MaxRegion      18      3    Control                                            ResetPickResult                                                                               18      4    Command                                            MinHitRegion   18      5    Command                                            MaxHitRegion   18      6    Command                                            PickResult     18      7    Command                                            Sync           18      8    Command                                   ______________________________________                                    

This format allows up to 16 registers within a group to be loaded while still only specifying a single address tag description word.

If the Mode of the address tag description word is set to indexed mode, then the high-order 16 bits are used as a mask to indicate which registers within the group are to be used. The bottom 4 bits of the address tag description word are unused. The group is specified by bits 4 to 8. Each bit in the mask is used to represent a unique tag within the group. If a bit is set then the corresponding register will be loaded. The number of bits set in the mask determines the number of data words that should be following the tag description word in the DMA buffer. The data is stored in order of increasing corresponding address tag.

DMA Buffer Addresses

Host software must generate the correct DMA buffer address for the GLINT DMA controller. Normally, this means that the address passed to GLINT must be the physical address of the DMA buffer in host memory. The buffer must also reside at contiguous physical addresses as accessed by GLINT. On a system which uses virtual memory for the address space of a task, some method of allocating contiguous physical memory, and mapping this into the address space of a task, must be used.

If the virtual memory buffer maps to non-contiguous physical memory, then the buffer must be divided into sets of contiguous physical memory pages and each of these sets transferred separately. In such a situation the whole DMA buffer cannot be transferred in one go; the host software must wait for each set to be transferred. Often the best way to handle these fragmented transfers is via an interrupt handler.

DMA Interrupts

GLINT provides interrupt support, as an alternative means of determining when a DMA transfer is complete. If enabled, the interrupt is generated whenever the DMACount register changes from having a non-zero to having a zero value. Since the DMACount register is decremented every time a data item is transferred from the DMA buffer this happens when the last data item is transferred from the DMA buffer.

To enable the DMA interrupt, the DMAInterruptEnable bit must be set in the IntEnable register. The interrupt handler should check the DMAFIag bit in the IntFlags register to determine that a DMA interrupt has actually occurred. To clear the interrupt a word should be written to the IntFlags register with the DMAFlag bit set to one.

This scheme frees the processor for other work while DMA is being completed. Since the overhead of handling an interrupt is often quite high for the host processor, the scheme should be tuned to allow a period of polling before sleeping on the interrupt.

Output FIFO and Graphics Processor FIFO Interface

To read data back from GLINT an output FIFO is provided. Each entry in this FIFO is 32-bits wide and it can hold tag or data values. Thus its format is unlike the input FIFO whose entries are always tag/data pairs (we can think of each entry in the input FIFO as being 41 bits wide: 9 bits for the tag and 32 bits for the data). The type of data written by GLINT to the output FIFO is controlled by the FilterMode register. This register allows filtering of output data in various categories including the following:

•Depth: output in this category results from an image upload of the Depth buffer.

•Stencil: output in this category results from an image upload of the Stencil buffer.

•Color: output in this category results from an image upload of the framebuffer.

•Synchronization: synchronization data is sent in response to a Sync command.

The data for the FilterMode register consists of 2 bits per category. If the least significant of these two bits is set (0×1) then output of the register tag for that category is enabled; if the most significant bit is set (0×2) then output of the data for that category is enabled. Both tag and data output can be enabled at the same time. In this case the tag is written first to the FIFO followed by the data.

For example, to perform an image upload from the framebuffer, the FilterMode register should have data output enabled for the Color category. Then, the rectangular area to be uploaded should be described to the rasterizer. Each pixel that is read from the framebuffer will then be placed into the output FIFO. If the output FIFO becomes full, then GLINT will block internally until space becomes available. It is the programmer's responsibility to read all data from the output FIFO. For example, it is important to know how many pixels should result from an image upload and to read exactly this many from the FIFO.

To read data from the output FIFO the OutputFIFOWords register should first be read to determine the number of entries in the FIFO (reading from the FIFO when it is empty returns undefined data). Then this many 32-bit data items are read from the FIFO. This procedure is repeated until all the expected data or tag items have been read. The address of the output FIFO is described below.

Note that all expected data must be read back. GLINT will block if the FIFO becomes full. Programmers must be careful to avoid the deadlock condition that will result if the host is waiting for space to become free in the input FIFO while GLINT is waiting for the host to read data from the output FIFO.

Graphics Processor FIFO Interface

GLINT has a sequence of 1K×32 bit addresses in the PCI Region 0 address map called the Graphics Processor FIFO Interface. To read from the output FIFO any-address in this range can be read (normally a program will choose the first address and use this as the address for the output FIFO). All 32-bit addresses in this region perform the same function: the range of addresses is provided for data transfer schemes which force the use of incrementing addresses.

Writing to a location in this address range provides raw access to the input FIFO. Again, the first address is normally chosen. Thus the same address can be used for both input and output FIFOs. Reading gives access to the output FIFO; writing gives access to the input FIFO.

Writing to the input FIFO by this method is different from writing to the memory mapped register file. Since the register file has a unique address for each register, writing to this unique address allows GLINT to determine the register for which the write is intended. This allows a tag/data pair to be constructed and inserted into the input FIFO. When writing to the raw FIFO address an address tag description must first be written followed by the associated data. In fact, the format of the tag descriptions and the data that follows is identical to that described above for DMA buffers. Instead of using the GLINT DMA it is possible to transfer data to GLINT by constructing a DMA-style buffer of data and then copying each item in this buffer to the raw input FIFO address. Based on the tag descriptions and data written GLINT constructs tag/data pairs to enter as real FIFO entries. The DMA mechanism can be thought of as an automatic way of writing to the raw input FIFO address.

Note, that when writing to the raw FIFO address the FIFO full condition must still be checked by reading the InFIFOSpace register. However, writing tag descriptions does not cause any entries to be entered into the FIFO: such a write simply establishes a set of tags to be paired with the subsequent data. Thus, free space need be ensured only for actual data items that are written (not the tag values). For example, in the simplest case where each tag is followed by a single data item, assuming that the FIFO is empty, then 32 writes are possible before checking again for free space.

Other Interrupts

GLINT also provides interrupt facilities for the following:

•Sync: If a Sync command is sent and the Sync interrupt has been enabled then once all rendering has been completed, a data value is entered into the Host Out FIFO, and a Sync interrupt is generated when this value reaches the output end of the FIFO. Synchronization is described further in the next section.

•External: this provides the capability for external hardware on a GLINT board (such as an external video timing generator) to generate interrupts to the host processor.

•Error: if enabled the error interrupt will occur when GLINT detects certain error conditions, such as an attempt to write to a full FIFO.

•Vertical Retrace: if enabled a vertical retrace interrupt is generated at the start of the video blank period.

Each of these are enabled and cleared in a similar way to the DMA interrupt.

Synchronization

There are three main cases where the host must synchronize with GLINT:

•before reading back from registers

•before directly accessing the framebuffer or the localbuffer via the bypass mechanism

•framebuffer management tasks such as double buffering

Synchronizing with GLINT implies waiting for any pending DMA to complete and waiting for the chip to complete any processing currently being performed. The following pseudo-code shows the general scheme:

    ______________________________________                                         GLINTData data;                                                                // wait for DMA to complete                                                    while (*DMACount != 0) {                                                          poll or wait for interrupt                                                  while (*InFIFOSpace < 2) {                                                        ;  // wait for free space in the FIFO                                       }                                                                              // enable sync output and send the Sync command                                data.Word = 0;                                                                 data.FilterMode.Synchronization = 0×1;                                   FilterMode(data.Word);                                                         Sync(0×0);                                                               /* wait for the sync outpyt data */                                            do {                                                                              while (*OutFIFOWords ==0)                                                         ;  // poll waiting for data in output                                    FIFO                                                                           } while (*OutputFIFO != Sync.sub.-- tag);                                      ______________________________________                                    

Initially, we wait for DMA to complete as normal. We then have to wait for space to become free in the FIFO (since the DMA controller actually loads the FIFO). We need space for 2 registers: one to enable generation of an output sync value, and the Sync command itself. The enable flag can be set at initialization time. The output value will be generated only when a Sync command has actually been sent, and GLINT has then completed all processing.

Rather than polling it is possible to use a Sync interrupt as mentioned in the previous section. As well as enabling the interrupt and setting the filter mode, the data sent in the Sync command must have the most significant bit set in order to generate the interrupt. The interrupt is generated when the tag or data reaches the output end of the Host Out FIFO. Use of the Sync interrupt has to be considered carefully as GLINT will generally empty the FIFO more quickly than it takes to set up and handle the interrupt.

Host Framebuffer Bypass

Normally, the host will access the framebuffer indirectly via commands sent to the GLINT FIFO interface. However, GLINT does provide the whole framebuffer as part of its address space so that it can be memory mapped by an application. Access to the framebuffer via this memory mapped route is independent of the GLINT FIFO.

Drivers may choose to use direct access to the framebuffer for algorithms which are not supported by GLINT. The framebuffer bypass supports big-endian, little-endian and GIB-endian formats.

A driver making use of the framebuffer bypass mechanism should synchronize framebuffer accesses made through the FIFO with those made directly through the memory map. If data is written to the FIFO and then an access is made to the framebuffer, it is possible that the framebuffer access will occur before the commands in the FIFO have been fully processed. This lack of temporal ordering is generally not desirable.

Framebuffer Dimensions and Depth

At reset time the hardware stores the size of the framebuffer in the FBMemoryControl register. This register can be read by software to determine the amount of VRAM on the display adapter. For a given amount of VRAM, software can configure different screen resolutions and off-screen memory regions.

The framebuffer width must be set up in the FBReadMode register. The first 9 bits of this register define 3 partial products which determine the offset in pixels from one scanline to the next. Typically, these values will be worked out at initialization time and a copy kept in software. When this register needs to be modified the software copy is retrieved and any other bits modified before writing to the register.

Once the offset from one scanline to the next has been established, determining the visible screen width and height becomes a clipping issue. The visible screen width and height are set up in the ScreenSize register and enabled by setting the ScreenScissorEnable bit in the ScissorMode register.

The framebuffer depth (8, 16 or 32-bit) is controlled by the FBModeSel register. This register provides a 2 bit field to control which of the three pixel depths is being used. The pixel depth can be changed at any time but this should not be attempted without first synchronizing with GLINT. The FBModeSel register is not a FIFO register and is updated immediately it is written. If GLINT is busy performing rendering operations, changing the pixel depth will corrupt that rendering.

Normally, the pixel depth is set at initialization time. To optimize certain 2D rendering operations it may be desirable to change it at other times. For example, if the pixel depth is normally 8 (or 16) bits, changing the pixel depth to 32 bits for the duration of a bitblt can quadruple (or double) the blt speed, when the blt source and destination edges are aligned on 32 bit boundaries. Once such a blt sequence has been set up the host software must wait and synchronize with GLINT and then reset the pixel depth before continuing with further rendering. It is not possible to change the pixel depth via the FIFO, thus explicit synchronization must always be used.

Host Localbuffer Bypass

As with the framebuffer, the localbuffer can be mapped in and accessed directly. The host should synchronize with GLINT before making any direct access to the localbuffer.

At reset time the hardware saves the size of the localbuffer in the LBMemoryControl register (localbuffer visible region size). In bypass mode the number of bits per pixel is either 32 or 64. This information is also set in the LBMemoryControl register (localbuffer bypass packing). This pixel packing defines the memory offset between one pixel and the next. A further set of 3 bits (localbuffer width) in the LBMemoryControl register defines the number of valid bits per pixel. A typical localbuffer configuration might be 48 bits per pixel but in bypass mode the data for each pixel starts on a 64-bit boundary. In this case valid pixel data will be contained in bits 0 to 47. Software must set the LBReadFormat register to tell GLINT how to interpret these valid bits.

Host software must set the width in pixels of each scanline of the localbuffer in the LBReadMode FIFO register. The first 9 bits of this register define 3 partial products which determine the offset in pixels from one scanline to the next. As with the framebuffer partial products, these values will usually be worked out at initialization time and a copy kept in software. When this register needs to be modified the software copy is retrieved and any other bits modified before writing to the register. If the system is set up so that each pixel in the framebuffer has a corresponding pixel in the localbuffer then this width will be the same as that set for the framebuffer.

The localbuffer is accessible via Regions 1 and 3 of the PCI address map for GLINT. The localbuffer bypass supports big-endian and little-endian formats. These are described in a later section.

Register Read Back

Under some operating environments, multiple tasks will want access to the GLINT chip. Sometimes a server task or driver will want to arbitrate access to GLINT on behalf of multiple applications. In these circumstances, the state of the GLINT chip may need to be saved and restored on each context switch. To facilitate this, the GLINT control registers can be read back. (However, internal and command registers cannot be read back.)

To perform a context switch the host must first synchronize with GLINT. This means waiting for outstanding DMA to complete, sending a Sync command and waiting for the sync output data to appear in the output FIFO. After this the registers can be read back.

To read a GLINT register the host reads the same address which would be used for a write, i.e. the base address of the register file plus the offset value for the register.

Note that since internal registers cannot be read back care must be taken when context switching a task which is making use of continue-draw commands. Continue-draw commands rely on the internal registers maintaining previous state. This state will be destroyed by any rendering work done by a new task. To prevent this, continue-draw commands should be performed via DMA since the context switch code has to wait for outstanding DMA to complete. Alternatively, continue-draw commands can be performed in a non-preemptable code segment.

Normally, reading back individual registers should be avoided. The need to synchronize with the chip can adversely affect performance. It is usually more appropriate to keep a software copy of the register which is updated when the actual register is updated.

Byte Swapping

Internally GLINT operates in little-endian mode. However, GLINT is designed to work with both big- and little-endian host processors. Since the PCIBus specification defines that byte ordering is preserved regardless of the size of the transfer operation, GLINT provides facilities to handle byte swapping. Each of the Configuration Space, Control Space, Framebuffer Bypass and Localbuffer Bypass memory areas have both big and little endian mappings available. The mapping to use typically depends on the endian ordering of the host processor.

The Configuration Space may be set by a resistor in the board design to be either little endian or big endian.

The Control Space in PCI address region 0, is 128K bytes in size, and consists of two 64K sized spaces. The first 64K provides little endian access to the control space registers; the second 64K provides big endian access to the same registers.

The framebuffer bypass consists of two PCI address regions: Region 2 and Region 4. Each is independently configurable to by the Aperture0 and Aperture1 control registers respectively, to one of three modes: no byte swap, 16-bit swap, full byte swap. Note that the 16 bit mode is needed for the following reason. If the framebuffer is configured for 16-bit pixels and the host is big-endian then simply byte swapping is not enough when a 32-bit access is made (to write two pixels). In this case, the required effect is that the bytes are swapped within each 16-bit word, but the two 16-bit halves of the 32-bit word are not swapped. This preserves the order of the pixels that are written as well as the byte ordering within each pixel. The 16 bit mode is referred to as GIB-endian in the PCI Multimedia Design Guide, version 1.0.

The localbuffer bypass consists of two PCI address regions: Region 1 and Region 3. Each is independently configurable to by the Aperture0 and Aperture1 control registers respectively, to one of two modes: no byte swap, full byte swap.

To save on the size of the address space required for GLINT, board vendors may choose to turn off access to the big endian regions (3 and 4) by the use of resistors on the board.

There is a bit available in the DMAControl control register to enable byte swapping of DMA data. Thus for big-endian hosts, this control bit would normally be enabled.

Red and Blue Swapping

For a given graphics board the RAMDAC and/or API will usually force a given interpretation for true color pixel values. For example, 32-bit pixels will be interpreted as either ARGB (alpha at byte 3, red at byte 2, green at byte 1 and blue at byte 0) or ABGR (blue at byte 2 and red at byte 0). The byte position for red and blue may be important for software which has been written to expect one byte order or the other, in particular when handling image data stored in a file.

GLINT provides two registers to specify the byte positions of blue and red internally. In the Alpha Blend Unit the AlphaBlendMode register contains a 1-bit field called ColorOrder. If this bit is set to zero then the byte ordering is ABGR; if the bit is set to one then the ordering is ARGB. As well as setting this bit in the Alpha Blend unit, it must also be set in the Color Formatting unit. In this unit the DitherMode register contains a Color Order bit with the same interpretation. The order applies to all of the true color pixel formats, regardless of the pixel depth.

Hardware Data Structures

Some of the hardware data structure implementations used in the presently preferred embodiment will now be described in detail. Of course these examples are provided merely to illustrate the presently preferred embodiment in great detail, and do not necessarily delimit any of the claimed inventions.

Localbuffer

The localbuffer holds the per pixel information corresponding to each displayed pixel and any texture maps. The per pixel information held in the localbuffer are Graphic ID (GID), Depth, Stencil and Frame Count Planes (FCP). The possible formats for each of these fields, and their use are covered individually in the following sections.

The maximum width of the localbuffer is 48 bits, but this can be reduced by changing the external memory configuration, albeit at the expense of reducing the functionality or dynamic range of one or more of the fields.

The localbuffer memory can be from 16 bits (assuming a depth buffer is always needed) to 48 bits wide in steps of 4 bits. The four fields supported in the localbuffer, their allowed lengths and positions are shown in the following table:

    ______________________________________                                         Field       Lengths   Start bit positions                                      ______________________________________                                         Depth       16, 24, 32                                                                               0                                                        Stencil     0, 4, 8   16, 20, 24, 28, 32                                       FrameCount  0, 4, 8   16, 20, 24, 28, 32, 36, 40                               GID         0, 4      16, 20, 24, 28, 32, 36, 40, 44, 48                       ______________________________________                                    

The order of the fields is as shown with the depth field at the least significant end and GID field at the most significant end. The GID is at the most significant end so that various combinations of the Stencil and FrameCount field widths can be used on a per window basis without the position of the GID fields moving. If the CID field is in a different positions in different windows then the ownership tests become impossible to do.

The GID, FrameCount, Stencil and Depth fields in the localbuffer are converted into the internal format by right justification if they are less than their internal widths, i.e. the unused bits are the most significant bits and they are set to 0.

The format of the localbuffer is specified in two places: the LBReadFormat register and the LBWriteFormat register.

It is still possible to part populate the localbuffer so other combinations of the field widths are possible (i.e. depth field width of 0), but this may give problems if texture maps are to be stored in the localbuffer as well.

Any non-bypass read or write to the localbuffer always reads or writes all 48 bits simultaneously.

GID field

The 4 bit GID field is used for pixel ownership tests to allow per pixel window clipping. Each window using this facility is assigned one of the GID values, and the visible pixels in the window have their GID field set to this value. If the test is enabled the current GID (set to correspond with the current window) is compared with the GID in the localbuffer for each fragment. If they are equal this pixel belongs to the window so the localbuffer and framebuffer at this coordinate may be updated.

Using the GID field for pixel ownership tests is optional and other methods of achieving the same result are:

•clip the primitive to the window's boundary (or rectangular tiles which make up the window's area) and render only the visible parts of the primitive

•use the scissor test to define the rectangular tiles which make up the window's visible area and render the primitive once per tile (This may be limited to only those tiles which the primitive intersects).

Depth Field

The depth field holds the depth (Z) value associated with a pixel and can be 16, 24 or 32 bits wide.

Stencil Field

The stencil field holds the stencil value associated with a pixel and can be 0, 4 or 8 bits wide.

The width of the stencil buffer is also stored in the StencilMode register and is needed for clamping and masking during the update methods. The stencil compare mask should be set up to exclude any absent bits from the stencil compare operation.

FrameCount Field

The Frame Count Field holds the frame count value associated with a pixel and can be 0, 4 or 8 bits wide. It is used during animation to support a fast clear mechanism to aid the rapid clearing of the depth and/or stencil fields needed at the start of each frame.

In addition to the fast clear mechanism the extent of all updates to the localbuffer and framebuffer can be recorded (MinRegion and MaxRegion registers) and read back (MinHitRegion and MaxHitRegion commands) to give the bounding box of the smallest area to clear. For some applications this will be significantly smaller than the whole window or screen, and hence faster.

The fast clear mechanism provides a method where the cost of clearing the depth and stencil buffers can be amortized over a number of clear operations issued by the application. This works as follows:

The window is divided up into n regions, where n is the range of the frame counter (16 or 256). Every time the application issues a clear command the reference frame counter is incremented (and allowed to roll over if it exceeds its maximum value) and the n^(th) region is cleared only. The clear updates the depth and/or stencil buffers to the new values and the frame count buffer with the reference value. This region is much smaller than the full window and hence takes less time to clear.

When the localbuffer is subsequently read and the frame count is found to be the same as the reference frame count (held in the Window register) the localbuffer data is used directly. However, if the frame count is found to be different from the reference frame count (held in the Window register) the data which would have been written, if the localbuffer had been cleared properly, is substituted for the stale data returned from the read. Any new writes to the localbuffer will set the frame count to the reference value so the next read on this pixel works normally without the substitution. The depth data to substitute is held in the FastClearDepth register and the stencil data to substitute is held in the StencilData register (along with other stencil information).

The fast clear mechanism does not present a total solution as the user can elect to clear just the stencil planes or just the depth planes, or both. The situation where the stencil planes only are `cleared` using the fast clear method, then some rendering is done and then the depth planes are `cleared` using the fast clear will leave ambiguous pixels in the localbuffer. The driver software will need to catch this situation, and fall back to using a per pixel write to do the second clear. Which field(s) the frame count plane refers to is recorded in the Window register.

When clear data is substituted for real memory data (during normal rendering operations) the depth write mask and stencil write masks are ignored to mimic the OpenGL operation when a buffer is cleared.

Localbuffer Coordinates

The coordinates generated by the rasterizer are 16 bit 2's complement numbers, and so have the range +32767 to -32768. The rasterizer will produce values in this range, but any which have a negative coordinate, or exceed the screen width or height (as programmed into the ScreenSize register) are discarded.

Coordinates can be defined window relative or screen relative and this is only relevant when the coordinate gets converted to an actual physical address in the localbuffer. In general it is expected that the windowing system will use absolute coordinates and the graphics system will use relative coordinates (to be independent of where the window really is).

GUI systems (such as Windows, Windows NT and X) usually have the origin of the coordinate system at the top left corner of the screen but this is not true for all graphics systems. For instance OpenGL uses the bottom left corner as its origin. The WindowOrigin bit in the LBReadMode register selects the top left (0) or bottom left (1) as the origin.

The actual equations used to calculate the localbuffer address to read and write are:

Bottom left origin:

Destination address=LBWindowBase-Y*W+X

Source address=

LBWindowBase-Y*W+X+LBSourceOffset

Top left origin:

Destination address=LBWindowBase+Y*W+X

Source address=

LBWindowBase+Y*W+X+LBSourceOffset

where:

X is the pixel's X coordinate.

Y is the pixel's Y coordinate.

LBWindowBase holds the base address in the localbuffer of the current window.

LBSourceOffset is normally zero except during a copy operation where data is read from one address and written to another address. The offset between source and destination is held in the LBSourceOffset register.

W is the screen width. Only a subset of widths are supported and these are encoded into the PP0, PP1 and PP2 fields in the LBReadMode register.

These address calculations translate a 2D address into a linear address.

The Screen width is specified as the sum of selected partial products so a full multiply operation is not needed. The partial products are selected by the fields PP0, PP1 and PP2 in the LBReadMode register.

For arbitrary width screens, for instance bitmaps in `off screen` memory, the next largest width from the table must be chosen. The difference between the table width and the bitmap width will be an unused strip of pixels down the right hand side of the bitmap.

Note that such bitmaps can be copied to the screen only as a series of scanlines rather than as a rectangular block. However, often windowing systems store offscreen bitmaps in rectangular regions which use the same stride as the screen. In this case normal bitblts can be used.

Texture Memory

The localbuffer is used to hold textures in the GLINT 300TX variant. In the GLINT 300SX variant the texture information is supplied by the host.

Framebuffer

The framebuffer is a region of memory where the information produced during rasterization is written prior to being displayed. This information is not restricted to color but can include window control data for LUT management and double buffering.

The framebuffer region can hold up to 32MBytes and there are very few restrictions on the format and size of the individual buffers which make up the video stream. Typical buffers include:

•True color or color index main planes,

•Overlay planes,

•Underlay planes,

•Window ID planes for LUT and double buffer management,

•Cursor planes.

Any combination of these planes can be supported up to a maximum of 32MBytes, but usually it is the video level processing which is the limiting factor. The following text examines the options and choices available from GLINT for rendering, copying, etc. data to these buffers.

To access alternative buffers either the FBPixelOffset register can be loaded, or the base address of the window held in the FBWindowBase register can be redefined. This is described in more detail below.

Buffer Organization

Each buffer resides at an address in the framebuffer memory map. For rendering and copying operations the actual buffer addresses can be on any pixel boundary. Display hardware will place some restrictions on this as it will need to access the multiple buffers in parallel to mix the buffers together depending on their relative priority, opacity and double buffer selection. For instance, visible buffers (rather than offscreen bitmaps) will typically need to be on a page boundary.

Consider the following highly configured example with a 1280×1024 double buffered system with 32 bit main planes (RGBA), 8 bit overlay and 4 bits of window control information (WID).

Combining the WID and overlay planes in the same 32 bit pixel has the advantage of reducing the amount of data to copy when a window moves, as only two copies are required--one for the main planes and one for the overlay and WID planes.

Note the position of the overlay and WID planes. This was not an arbitrary choice but one imposed by the (presumed) desire to use the color processing capabilities of GLINT (dither and interpolation) in the overlay planes. The conversion of the internal color format to the external one stored in the framebuffer depends on the size and position of the component. Note that GLINT does not support all possible configurations. For example; if the overlay and WID bits were swapped, then eight bit color index starting at bit 4 would be required to render to the overlay, but this is not supported.

Framebuffer Coordinates

Coordinate generation for the flamebuffer is similar to that for the localbuffer, but there are some key differences.

As was mentioned before, the coordinates generated by the rasterizer are 16 bit 2's complement numbers. Coordinates can be defined as window relative or screen relative, though this is only relevant when the coordinate gets convened to an actual physical address in the flamebuffer. The WindowOrigin bit in the FBReadMode register selects top left (0) or bottom left (1) as the origin for the flamebuffer.

The actual equations used to calculate the flamebuffer address to read and write are:

Bottom left origin:

Destination address=FBWindowBase-Y*W+X+FBPixelOffset

Source address=FBWindowBase-Y*W+X+FBPixelOffset+FBSourceOffset

Top left origin:

Destination address=FBWindowBase+Y*W+X+FBPixelOffset

Source address=FBWindowBase+Y*W+X+FBPixelOffset+FBSourceOffset

These address calculations translate a 2D address into a linear address, so non power of two framebuffer widths (i.e. 1280) are economical in memory.

The width is specified as the sum of selected partial products so a full multiply operation is not needed. The partial products are selected by the fields PP0, PP1 and PP2 in the FBReadMode register. This is the same mechanism as is used to set the width of the localbuffer, but the widths may be set independently.

For arbitrary screen sizes, for instance when rendering to `off screen` memory such as bitmaps the next largest width from the table must be chosen. The difference between the table width and the bitmap width will be an unused strip of pixels down the right hand side of the bitmap.

Note that such bitmaps can be copied to the screen only as a series of scanlines rather than as a rectangular block. However, often windowing systems store offscreen bitmaps in rectangular regions which use the same stride as the screen. In this case normal bitblts can be used.

Color Formats

The contents of the framebuffer can be regarded in two ways:

•As a collection of fields of up to 32 bits with no meaning or assumed format as far as GLINT is concerned. Bit planes may be allocated to control cursor, LUT, multi-buffer visibility or priority functions. In this case GLINT will be used to set and clear bit planes quickly but not perform any color processing such as interpolation or dithering. All the color processing can be disabled so that raw reads and writes are done and the only operations are write masking and logical ops. This allows the control planes to be updated and modified as necessary. Obviously this technique can also be used for overlay buffers, etc. providing color processing is not required.

•As a collection of one or more color components. All the processing of color components, except for the final write mask and logical ops are done using the internal color format of 8 bits per red, green, blue and alpha color channels. The final stage before write mask and logical ops processing converts the internal color format to that required by the physical configuration of the framebuffer and video logic. The nomenclature n@m means this component is n bits wide and starts at bit position m in the framebuffer. The least significant bit position is 0 and a dash in a column indicates that this component does not exist for this mode. The ColorOrder is specified by a bit in the DitherMode register.

Some important points to note:

•The alpha channel is always associated with the RGB color channels rather than being a separate buffer. This allows it to be moved in parallel and to work correctly in multi-buffer updates and double buffering. If the framebuffer is not configured with an alpha channel (e.g. 24 bit framebuffer width with 8:8:8:8 RGB format) then some of the rendering modes which use the retained alpha buffer cannot be used. In these cases the NoAlphaBuffer bit in the AlphaBlendMode register should be set so that an alpha value of 255 is substituted. For the RGB modes where no alpha channel is present (e.g. 3:3:2) then this substitution is done automatically.

•For the Front and Back modes the data value is replicated into both buffers.

•All writes to the framebuffer try to update all 32 bits irrespective of the color format. This may not matter if the memory planes don't exist, but if they are being used (as overlay planes, for example) then the write masks (FBSoftwareWriteMask or FBHardwareWriteMask) must be set up to protect the alternative planes.

•When reading the framebuffer RGBA components are scaled to their internal width of 8 bits, if needed for alpha blending.

CI values are left justified with the unused bits (if any) set to zero and are subsequently processed as the red component. The result is replicated into each of the streams G,B and A giving four copies for CI8 and eight copies for CI4.

•The 4:4:4:4 Front and Back formats are designed to support 12 bit double buffering with 4 bit Alpha, in a 32 bit system.

•The 3:3:2 Front and Back formats are designed to support 8 bit double buffering in a 16 bit system.

•The 1:2:1 Front and Back formats are designed to support 4 bit double buffering in an 8 bit system.

•It is possible to have a color index buffer at other positions as long as reduced functionality is acceptable. For example a 4 bit CI buffer at bit position 16 can be achieved using write masking and 4:4:4:4 Front format with color interpolation, but dithering is lost.

The format information needs to be stored in two places: the DitherMode register and the AlphaBlendMode register.

    ______________________________________                                                      Internal Color Channel                                            Format     Name    R        G     B     A                                      ______________________________________                                         Color  0       8:8:8:8 8@0    8@8    8@16 8@24                                 Order: 1       5:5:5:5 5@0    5@5    5@10 5@15                                 RGB    2       4:4:4:4 4@0    4@4   4@8   4@12                                        3       4:4:4:4 4@0    4@8    4@16 4@24                                                Front   4@4     4@12  4@20 4@28                                        4       4:4:4:4 4@0    4@8    4@16 4@24                                                Back    4@4     4@12  4@20 4@28                                        5       3:3:2   3@0    3@3   2@6   --                                                  Front   3@8     3@11  2@14                                             6       3:3:2   3@0    3@3   2@6   --                                                  Back    3@8     3@11  2@14                                             7       1:2:1   1@0    2@1   1@3   --                                                  Front   1@4    2@5   1@7                                               8       1:2:1   1@0    2@1   1@3   --                                                  Back    1@4    2@5   1@7                                        Color  0       8:8:8:8  8@16  8@8   8@0   8@24                                 Order: 1       5:5:5:5  5@10  5@5   5@0   5@15                                 BGR    2       4:4:4:4 4@8    4@4   4@0   4@12                                        3       4:4:4:4  4@16  4@8   4@0   4@24                                                Front    4@20   4@12 4@4   4@28                                        4       4:4:4:4  4@16  4@8   4@0   4@24                                                Back     4@20   4@12 4@4   4@28                                        5       3:3:2   3@5    3@2   2@0   --                                                  Front    3@13   3@10 2@8                                               6       3:3:2   3@5    3@2   2@0   --                                                  Back     3@13   3@10 2@8                                               7       1:2:1   1@3    2@1   1@0   --                                                  Front   1@7    2@5   1@4                                               8       1:2:1   1@3    2@1   1@0   --                                                  Back    1@7    2@5   1@4                                        CI     14      CI8     8@0    0     0     0                                           15      CI4     4@0    0     0     0                                    ______________________________________                                    

Overlays and Underlays

In a GUI system there are two possible relationships between the overlay planes (or underlay) and the main planes.

•The overlay planes are fixed to the main planes, so that if the window is moved then both the data in the main planes and overlay planes move together.

•The overlay planes are not fixed to the main planes but floating, so that moving a window only moves the associated main or overlay planes.

In the fixed case both planes can share the same GID. The pixel offset is used to redirect the reads and writes between the main planes and the overlay (underlay) buffer. The pixel ownership tests using the GID field in the localbuffer work as expected.

In the floating case different GIDs are the best choice, because the same GID planes in the localbuffer can not be used for pixel ownership tests. The alternatives are not to use the GID based pixel ownership tests for one of the buffers but rely on the scissor clipping, or to install a second set of CID planes so each buffer has it's own set. GLINT allows either approach.

If rendering operations to the main and overlay planes both need the depth or stencil buffers, and the windows in each overlap then each buffer will need its own exclusive depth and/or stencil buffers. This is easily achieved with GLINT by assigning different regions in the localbuffer to each of the buffers. Typically this would double the localbuffer memory requirements.

One scenario where the above two considerations do not cause problems, is when the overlay planes are used exclusively by the GUI system, and the main planes are used for the 3D graphics.

VRAM Modes

High performance systems will typically use VRAM for the framebuffer and the extended functionality of VRAM over DRAM can be used to enhance performance for many rendering tasks.

Hardware Write Masks

These allow write masking in the framebuffer without incurring a performance penalty. If hardware write masks are not available, GLINT must be programmed to read the memory, merge the value with the new value using the write mask, and write it back.

To use hardware write masking, the required write mask is written to the FBHardwareWriteMask register, the FBSoftwareWriteMask register should be set to all 1's, and the number of framebuffer reads is set to 0 (for normal rendering). This is achieved by clearing the ReadSource and ReadDestination enables in the FBReadMode register.

To use software write masking, the required write mask is written to the FBSoftwareWriteMask register and the number of framebuffer reads is set to 1 (for normal rendering). This is achieved by setting the ReadDestination enable in the FBReadMode register.

Block Writes

Block writes cause consecutive pixels in the framebuffer to be written simultaneously. This is useful when filling large areas but does have some restrictions:

•No pixel level clipping is available;

•No depth or stencil testing can be done;

•All the pixels must be written with the same value so no color interpolation, blending, dithering or logical ops can be done; and

•The area is defined in screen relative coordinates.

Block writes are not restricted to rectangular areas and can be used for any trapezoid. Hardware write masking is available during block writes.

The following registers need to be set up before block fills can be used:

FBBlockColor register with the value to write to each pixel; and

FBWriteMode register with the block width field.

Sending a Render command with the PrimitiveType field set to "trapezoid" and the FastFillEnable and FastFillIncrement fields set up will then cause block filling of the area. Note that during a block fill of a trapezoid any inappropriate state is ignored so even if color interpolation, depth testing and logical ops, for example, are enabled they have no effect.

The block sizes supported are 8, 16 and 32 pixels. GLINT takes care of filling any partial blocks at the end of spans.

Graphics Programming

GLINT provides a rich variety of operations for 2D and 3D graphics supported by its Pipelined architecture.

The Graphics Pipeline

This section describes each of the units in the graphics Pipeline. FIG. 2C shows a schematic of the pipeline. In this diagram, the localbuffer contains the pixel ownership values (known as Graphic IDs), the FrameCount Planes (FCP), Depth (Z) and Stencil buffer. The framebuffer contains the Red, Green, Blue and Alpha bitplanes. The operations in the Pipeline include:

•Rasterizer scan converts the given primitive into a series of fragments for processing by the rest of the pipeline.

•Scissor Test clips out fragments that lie outside the bounds of a user defined scissor rectangle and also performs screen clipping to stop illegal access outside the screen memory.

•Stipple Test masks out certain fragments according to a specified pattern. Line and area stipples are available.

•Color DDA is responsible for generating the color information (True Color RGBA or Color Index(CD) associated with a fragment.

•Texture is concerned with mapping a portion of a specified image (texture) onto a fragment. The process involves filtering to calculate the texture color, and application which applies the texture color to the fragment color.

•Fog blends a fog color with a fragment's color according to a given fog factor. Fogging is used for depth cuing images and to simulate atmospheric fogging.

•Antialias Application combines the incoming fragment's alpha value with its coverage value when antialiasing is enabled.

•Alpha Test conditionally discards a fragment based on the outcome of a comparison between the fragments alpha value and a reference alpha value.

•Pixel Ownership is concerned with ensuring that the location in the framebuffer for the current fragment is owned by the current visual. Comparison occurs between the given fragment and the Graphic ID value in the localbuffer, at the corresponding location, to determine whether the fragment should be discarded.

•Stencil Test conditionally discards a fragment based on the outcome of a test between the given fragment and the value in the stencil buffer at the corresponding location. The stencil buffer is updated dependent on the result of the stencil test and the depth test.

•Depth Test conditionally discards a fragment based on the outcome of a test between the depth value for the given fragment and the value in the depth buffer at the corresponding location. The result of the depth test can be used to control the updating of the stencil buffer.

•Alpha Blending combines the incoming fragment's color with the color in the framebuffer at the corresponding location.

•Color Formatting converts the fragment's color into the format in which the color information is stored in the framebuffer. This may optionally involve dithering.

The Pipeline structure of GLINT is very efficient at processing fragments, for example, texture mapping calculations are not actually performed on fragments that get clipped out by scissor testing. This approach saves substantial computational effort. The pipelined nature does however mean that when programming GLINT one should be aware of what all the pipeline stages are doing at any time. For example, many operations require both a read and/or write to the localbuffer and framebuffer; in this case it is not sufficient to set a logical operation to XOR and enable logical operations, but it is also necessary to enable the reading/writing of data from/to the framebuffer.

A Gouraud Shaded Triangle

We may now revisit the "day in the life of a triangle" example given above, and review the actions taken in greater detail. Again, the primitive being rendered will be a Gouraud shaded, depth buffered triangle. For this example assume that the triangle is to be drawn into a window which has its colormap set for RGB as opposed to color index operation. This means that all three color components; red, green and blue, must be handled. Also, assume the coordinate origin is bottom left of the window and drawing will be from top to bottom. GLINT can draw from top to bottom or bottom to top.

Consider a triangle with vertices, v₁, v₂ and v₃ where each vertex comprises X, Y and Z coordinates. Each vertex has a different color made up of red, green and blue (R, G and B) components. The alpha component will be omitted for this example.

Initialization

GLINT requires many of its registers to be initialized in a particular way, regardless of what is to be drawn, for instance, the screen size and appropriate clipping must be set up. Normally this only needs to be done once and for clarity this example assumes that all initialization has already been done.

Other state will change occasionally, though not usually on a per primitive basis, for instance enabling Gouraud shading and depth buffering.

Dominant and Subordinate Sides of a Triangle

As shown in FIG. 4A, the dominant side of a triangle is that with the greatest range of Y values. The choice of dominant side is optional when the triangle is either flat bottomed or flat topped.

GLINT always draws triangles starting from the dominant edge towards the subordinate edges. This simplifies the calculation of set up parameters as will be seen below.

These values allow the color of each fragment in the triangle to be determined by linear interpolation. For example, the red component color value of a fragment at X_(n),Y_(m) could be calculated by:

•adding dRdy₁₃, for each scanline between Y₁ and Y_(n), to R₁.

•then adding dRdx for each fragment along scanline Y_(n) from the left edge to X_(n).

The example chosen has the `knee,` i.e. vertex 2, on the right hand side, and drawing is from left to right. If the knee were on the left side (or drawing was from right to left), then the Y deltas for both the subordinate sides would be needed to interpolate the start values for each color component (and the depth value) on each scanline. For this reason GLINT always draws triangles starting from the dominant edge and towards the subordinate edges. For the example triangle, this means left to right.

Register Set Up for Color Interpolation

For the example triangle, the GLINT registers must be set as follows, for color interpolation. Note that the format for color values is 24 bit, fixed point 2's complement.

    ______________________________________                                         // Load the color start and delta values to draw                               // triangle                                                                    RStart (R.sub.1)                                                               GStart (G.sub.1)                                                               BStart (B.sub.1)                                                               dRdyDom (dRdy.sub.13)                                                                         // To walk up the dominant edge                                 dGdyDom (dGdy.sub.13)                                                          dBdyDom (dBdy.sub.13)                                                          dRdx    (dRdx)     // To walk along the scanline                               dGdx    (dGdx)                                                                 dBdx    (dBdx)                                                                 ______________________________________                                    

Calculating Depth Gradient Values

To draw from left to right and top to bottom, the depth gradients (or deltas) required for interpolation are: ##EQU1##

And from the plane equation: ##EQU2## where

    c=|(X.sub.1 -X.sub.3)(Y.sub.2 -Y.sub.3)-(X.sub.2 -X.sub.3)(Y.sub.1 -Y.sub.1)|

The divisor, shown here as c, is the same as for color gradient values. The two deltas dZdy₁₃ and dZdx allow the Z value of each fragment in the triangle to be determined by linear interpolation, just as for the color interpolation.

Register Set Up for Depth Testing

Internally GLINT uses fixed point arithmetic. Each depth value must be converted into a 2's complement 32.16 bit fixed point number and then loaded into the appropriate pair of 32 bit registers. The `Upper` or `U` registers store the integer portion, whilst the `Lower` or `L` registers store the 16 fractional bits, left justified and zero filled.

For the example triangle, GLINT would need its registers set up as follows:

    ______________________________________                                         // Load the depth start and delta values                                       // to draw a triangle                                                          ZStartU (Z1.sub.-- MS)                                                         ZStartL (Z1.sub.-- LS)                                                         dZdyDomU (dZdy13.sub.-- MS)                                                    dZdyDomL (dZdy13.sub.-- LS)                                                    dZdxU (dZdx.sub.-- MS)                                                         dZdxL (dZdx.sub.-- LS)                                                         ______________________________________                                    

Calculating the Slopes for each Side

GLINT draws filled shapes such as triangles as a series of spans with one span per scanline. Therefore it needs to know the start and end X coordinate of each span. These are determined by `edge walking`. This process involves adding one delta value to the previous span's start X coordinate and another delta value to the previous span's end x coordinate to determine the X coordinates of the new span. These delta values are in effect the slopes of the triangle sides. To draw from left to right and top to bottom, the slopes of the three sides are calculated as: ##EQU3##

This triangle will be drawn in two parts, top down to the `knee` (i.e. vertex 2), and then from there to the bottom. The dominant side is the left side so for the top half:

    dXDom=dX.sub.13

    dXSub=dX.sub.12

The start X,Y, the number of scanlines, and the above deltas give GLINT enough information to edge walk the top half of the triangle. However, to indicate that this is not a flat topped triangle (GLINT is designed to rasterize screen aligned trapezoids and flat topped triangles), the same start position in terms of X must be given twice as StartXDom and StartXSub.

To edge walk the lower half of the triangle, selected additional information is required. The slope of the dominant edge remains unchanged, but the subordinate edge slope needs to be set to:

    dXSub=dX.sub.23

Also the number of scanlines to be covered from Y₂ to Y₃ needs to be given. Finally to avoid any rounding errors accumulated in edge walking to X₂ (which can lead to pixel errors), StartXSub must be set to X₂.

Rasterizer Mode

The GLINT rasterizer has a number of modes which have effect from the time they are set until they are modified and can thus affect many primitives. In the case of the Gouraud shaded triangle the default value for these modes are suitable.

Subpixel Correction

GLINT can perform subpixel correction of all interpolated values when rendering aliased trapezoids. This correction ensures that any parameter (color/depth/texture/fog) is correctly sampled at the center of a fragment. Subpixel correction will generally always be enabled when rendering any trapezoid which is smooth shaded, textured, fogged or depth buffered. Control of subpixel correction is in the Render command register described in the next section, and is selectable on a per primitive basis.

Rasterization

GLINT is almost ready to draw the triangle. Setting up the registers as described here and sending the Render command will cause the top half of the example triangle to be drawn.

For drawing the example triangle, all the bit fields within the Render command should be set to 0 except the PrimitiveType which should be set to trapezoid and the SubPixelCorrectionEnable bit which should be set to TRUE.

    ______________________________________                                         // Draw triangle with knee                                                     // Set deltas                                                                  StartXDom (X.sub.1 <<16) // Converted to 16.16 fixed                           point                                                                          dXDom (((X.sub.3 - X.sub.1)<<16) / (Y.sub.3 - Y.sub.1))                        StartXSub (X.sub.1 <<16)                                                       dXSubv (((X.sub.2 - X.sub.1)<<16) / (Y.sub.2 - Y.sub.1))                       StartY (Y.sub.1 <<16)                                                          dY (-1<<16)                                                                    Count (Y.sub.1 - Y.sub.2)                                                      // Set the render command mode                                                 render.PrimitiveType = GLINT.sub.-- TRAPEZOID.sub.-- PRIMITIVE                 render.SubPixelCorrectionEnable = TRUE                                         // Draw the top half of the triangle                                           Render(render)                                                                 ______________________________________                                    

After the Render command has been issued, the registers in GLINT can immediately be altered to draw the lower half of the triangle. Note that only two registers need be loaded and the command ContinueNewSub sent. Once GLINT has received ContinueNewSub, drawing of this sub-triangle will begin.

    ______________________________________                                         // Setup the delta and start for the new edge                                  StartXSub (X.sub.2 <<16)                                                       dXSub (((X.sub.3 - X.sub.2)<<16) / (Y.sub.3 - Y.sub.2))                        // Draw sub-triangle                                                           ContinueNewSub (Y.sub.2 - Y.sub.3) // Draw lower half                          ______________________________________                                    

Rasterizer Unit

The rasterizer decomposes a given primitive into a series of fragments for processing by the rest of the Pipeline.

GLINT can directly rasterize:

•aliased screen aligned trapezoids

•aliased single pixel wide lines

•aliased single pixel points

•antialiased screen aligned trapezoids

•antialiased circular points

All other primitives are treated as one or more of the above, for example an antialiased line is drawn as a series of antialiased trapezoids.

Trapezoids

GLINT's basic area primitives are screen aligned trapezoids. These are characterized by having top and bottom edges parallel to the X axis. The side edges may be vertical (a rectangle), but in general will be diagonal. The top or bottom edges can degenerate into points in which case we are left with either flat topped or flat bottomed triangles. Any polygon can be decomposed into screen aligned trapezoids or triangles. Usually, polygons are decomposed into triangles because the interpolation of values over non-triangular polygons is ill defined. The rasterizer does handle flat topped and flat bottomed `bow tie` polygons which are a special case of screen aligned trapezoids.

To render a triangle, the approach adopted to determine which fragments are to be drawn is known as `edge walking`. Suppose the aliased triangle shown in FIG. 4A was to be rendered from top to bottom and the origin was bottom left of the window. Starting at (X1, Y1) then decrementing Y and using the slope equations for edges 1-2 and 1-3, the intersection of each edge on each scanline can be calculated. This results in a span of fragments per scanline for the top trapezoid. The same method can be used for the bottom trapezoid using slopes 2-3 and 1-3.

It is usually required that adjacent triangles or polygons which share an edge or vertex are drawn such that pixels which make up the edge or vertex get drawn exactly once. This may be achieved by omitting the pixels down the left or the right sides and the pixels along the top or lower sides. GLINT has adopted the convention of omitting the pixels down the right hand edge. Control of whether the pixels along the top or lower sides are omitted depends on the start Y value and the number of scanlines to be covered. With the example, if StartY=Y1 and the number of scanlines is set to Y1-Y2, the lower edge of the top half of the triangle will be excluded. This excluded edge will get drawn as part of the lower half of the triangle.

To minimize delta calculations, triangles may be scan converted from left to right or from right to left. The direction depends on the dominant edge, that is the edge which has the maximum range of Y values. Rendering always proceeds from the dominant edge towards the relevant subordinate edge. In the example above, the dominant edge is 1-3 so rendering will be from right to left.

The sequence of actions required to render a triangle (with a `knee`) is:

•Load the edge parameters and derivatives for the dominant edge and the first subordinate edges in the first triangle.

•Send the Render command. This starts the scan conversion of the first triangle, working from the dominant edge. This means that for triangles where the knee is on the left we are scanning right to left, and vice versa for triangles where the knee is on the right.

•Load the edge parameters and derivatives for the remaining subordinate edge in the second triangle.

•Send the ContinueNewSub command. This starts the scan conversion of the second triangle.

Pseudocode for the above example is:

    ______________________________________                                         // Set the rasterizer mode to the default                                      RasterizerMode (0)                                                             // Setup the start values and the deltas.                                      // Note that the X and Y coordinates are converted                             // to 16.16 format                                                             StartXDom (X1<<16)                                                             dXDom (((X3 - X1)<<16) / (Y3 - Y1))                                            StartXSub (X1<<16)                                                             dXSub (((X2- X1)<<16) / (Y2 - Y1))                                             StartY (Y1<<16)                                                                dY (-1<<16)     // Down the screen                                             Count (Y1 - Y2)                                                                // Set the render mode to aliased primitive with                               // subpixel correction.                                                        render.PrimitiveType = GLINT.sub.-- TRAPEZOID.sub.-- PRIMITIVE                 render.SubpixelCorrectionEnable = GLINT.sub.-- TRUE                            render.AntialiasEnable = GLINT.sub.-- DISABLE                                  // Draw top half of the triangle                                               Render(render)                                                                 // Set the start and delta for the second half of                              // the triangle                                                                StartXSub (X2<<16)                                                             dXSub (((X3- X2)<<16) / (Y3 - Y2))                                             // Draw lower half of triangle                                                 ContinueNewSub (abs(Y2 - Y3))                                                  ______________________________________                                    

After the Render command has been sent, the registers in GLINT can immediately be altered to draw the second half of the triangle. For this, note that only two registers need be loaded and the command ContinueNewSub be sent. Once drawing of the first triangle is complete and GLINT has received the ContinueNewSub command, drawing of this sub-triangle will start. The ContinueNewSub command register is loaded with the remaining number of scanlines to be rendered.

Lines

Single pixel wide aliased lines are drawn using a DDA algorithm, so all GLINT needs by way of input data is StartX, StartY, dX, dY and length.

For polylines, a ContinueNewLine command (analogous to the Continue command used at the knee of a triangle) is used at vertices.

When a Continue command is issued some error will be propagated along the line. To minimize this, a choice of actions are available as to how the DDA units are restarted on the receipt of a Continue command. It is recommended that for OpenGL rendering the ContinueNewLine command is not used and individual segments are rendered.

Antialiased lines, of any width, are rendered as antialiased screen-aligned trapezoids.

Points

GLINT supports a single pixel aliased point primitive. For points larger than one pixel trapezoids should be used. In this case the PrimitiveType field in the Render command should be set to equal GLINT₋₋ POINT₋₋ PRIMITIVE.

Antialiasing

GLINT uses a subpixel point sampling algorithm to antialias primitives. GLINT can directly rasterize antialiased trapezoids and points. Other primitives are composed from these base primitives.

The rasterizer associates a coverage value with each fragment produced when antialiasing. This value represents the percentage coverage of the pixel by the fragment. GLINT supports two levels of antialiasing quality:

•normal, which represents 4×4 pixel subsampling

•high, which represents 8×8 pixel subsampling.

Selection between these two is made by the AntialiasingQuality bit within the Render command register.

When rendering antialiased primitives with GLINT the FlushSpan command is used to terminate rendering of a primitive. This is due to the nature of GLINT antialiasing. When a primitive is rendered which does not happen to complete on a scanline boundary, GLINT retains antialiasing information about the last sub-scanline(s) it has processed, but does not generate fragments for them unless a FlushSpan command is received. The commands ContinueNewSub, ContinueNewDom or Continue can then be used, as appropriate, to maintain continuity between adjacent trapezoids. This allows complex antialiased primitives to be built up from simple trapezoids or points.

To illustrate this consider using screen aligned trapezoids to render an antialiased line. The line will in general consist of three screen aligned trapezoids as shown in FIG. 4B. This Figure illustrates the sequence of rendering an Antialiased Line primitive. Note that the line has finite width.

    ______________________________________                                         // Setup the blend and coverage application units                              // as appropriate - not shown                                                  // In this example only the edge deltas are shown                              // loaded into registers for clarity. In reality                               // start X and Y values are required                                           // Render Trapezoid A                                                          dY(1<<16)                                                                      dXDom(dXDom1<<16)                                                              dXSub(dXSub1<<16)                                                              Count(count1)                                                                  render.PrimitiveType = GLINT.sub.-- TRAPEZOID                                  render.AntialiasEnable = GLINT.sub.-- TRUE                                     render.AntialiasQuality = GLINT.sub.-- MIN.sub.-- ANTIALIAS                    render.CoverageEnable = GLINT.sub.-- TRUE                                      Render(render)                                                                 // Render Trapezoid B                                                          dXSub(dXSub2<<16)                                                              ContinueNewSub(count2)                                                         // Render Trapezoid C                                                          dXDom(dXDom2<<16)                                                              ContinueNewDom(count3)                                                         // Now we have finished the primitive flush out                                // the last scanline                                                           FlushSpan( )                                                                   ______________________________________                                    

Note that when rendering antialiased primitives, any count values should be given in subscanlines, for example if the quality is 4×4 then any scanline count must be multiplied by 4 to convert it into a subscanline count. Similarly, any delta value must be divided by 4.

When rendering, AntialiasEnable must be set in the AntialiasMode register to scale the fragments color by the coverage value. An appropriate blending function should also be enabled.

Note, when rendering antialiased bow-ties, the coverage value on the cross-over scanline may be incorrect.

GLINT can render small antialiased points. Antialiased points are treated as circles, with the coverage of the boundary fragments ranging from 0% to 100%. GLINT supports:

•point radii of 0.5 to 16.0 in steps of 0.25 for 4×4 antialiasing

•point radii of 0.25 to 8.0 in steps of 0.125 for 8×8 antialiasing

To scan convert an antialiased point as a circle, GLINT traverses the boundary in sub scanline steps to calculate the coverage value. For this, the sub-scanline intersections are calculated incrementally using a small table. The table holds the change in X for a step in Y. Symmetry is used so the table only holds the delta values for one quadrant.

StartXDom, StartXSub and StartY are set to the top or bottom of the circle and dY set to the subscanline step. In the case of an even diameter, the last of the required entries in the table is set to zero.

Since the table is configurable, point shapes other than circles can be rendered. Also if the StartXDom and StartXSub values are not coincident then horizontal thick lines with rounded ends, can be rendered.

Block Write Operation

GLINT supports VRAM block writes with block sizes of 8, 16 and 32 pixels. The block write method does have some restrictions: None of the per pixel clipping, stipple, or fragment operations are available with the exception of write masks. One subtle restriction is that the block coordinates will be interpreted as screen relative and not window relative when the pixel mask is calculated in the Framebuffer Units.

Any screen aligned trapezoid can be filled using block writes, not just rectangles.

The use of block writes is enabled by setting the FastFillEnable and FastFillIncrement fields in the Render command register. The framebuffer write unit must also be configured.

Note only the Rasterizer, Framebuffer Read and Framebuffer Write units are involved in block filling. The other units will ignore block write fragments, so it is not necessary to disable them.

Sub Pixel Precision and Correction

As the rasterizer has 16 bits of fraction precision, and the screen width used is typically less than 2¹⁶ wide a number of bits called subpixel precision bits, are available. Consider a screen width of 4096 pixels. This figure gives a subpixel precision of 4 bits (4096=2¹²). The extra bits are required for a number of reasons:

•antialiasing (where vertex start positions can be supplied to subpixel precision)

•when using an accumulation buffer (where scans are rendered multiple times with jittered input vertices)

•for correct interpolation of parameters to give high quality shading as described below

GLINT supports subpixel correction of interpolated values when rendering aliased trapezoids. Subpixel correction ensures that all interpolated parameters associated with a fragment (color, depth, fog, texture) are correctly sampled at the fragment's center. This correction is required to ensure consistent shading of objects made from many primitives. It should generally be enabled for all aliased rendering which uses interpolated parameters.

Subpixel correction is not applied to antialiased primitives.

Bitmaps

A Bitmap primitive is a trapezoid or line of ones and zeros which control which fragments are generated by the rasterizer. Only fragments where the corresponding Bitmap bit is set are submitted for drawing. The normal use for this is in drawing characters, although the mechanism is available for all primitives. The Bitmap data is packed contiguously into 32 bit words so that rows are packed adjacent to each other. Bits in the mask word are by default used from the least significant end towards the most significant end and are applied to pixels in the order they are generated in.

The rasterizer scans through the bits in each word of the Bitmap data and increments the X,Y coordinates to trace out the rectangle of the given width and height. By default, any set bits (1) in the Bitmap cause a fragment to be generated, any reset bits (0) cause the fragment to be rejected.

The selection of bits from the BitMaskPattern register can be mirrored, that is, the pattern is traversed from MSB to LSB rather than LSB to MSB. Also, the sense of the test can be reversed such that a set bit causes a fragment to be rejected and vice versa. This control is found in the RasterizerMode register.

When one Bitmap word has been exhausted and pixels in the rectangle still remain then rasterization is suspended until the next write to the BitMaskPattern register. Any unused bits in the last Bitmap word are discarded.

Image Copy/Upload/Download

GLINT supports three "pixel rectangle" operations: copy, upload and download. These can apply to the Depth or Stencil Buffers (held within the localbuffer) or the framebuffer.

It should be emphasized that the GLINT copy operation moves RAW blocks of data around buffers. To zoom or re-format data, in the presently preferred embodiment, external software must upload the data, process it and then download it again.

To copy a rectangular area, the rasterizer would be configured to render the destination rectangle, thus generating fragments for the area to be copied. GLINT copy works by adding a linear offset to the destination fragment's address to find the source fragment's address.

Note that the offset is independent of the origin of the buffer or window, as it is added to the destination address. Care must be taken when the source and destination overlap to choose the source scanning direction so that the overlapping area is not overwritten before it has been moved. This may be done by swapping the values written to the StartXDom and StartXSub, or by changing the sign of dY and setting StartY to be the opposite side of the rectangle.

Localbuffer copy operations are correctly tested for pixel ownership. Note that this implies two reads of the localbuffer, one to collect the source data, and one to get the destination GID for the pixel ownership test.

GLINT buffer upload/downloads are very similar to copies in that the region of interest is generated in the rasterizer. However, the localbuffer and framebuffer are generally configured to read or to write only, rather than both read and write. The exception is that an image load may use pixel ownership tests, in which case the localbuffer destination read must be enabled.

Units which can generate fragment values, the color DDA unit for example, should generally be disabled for any copy/upload/download operations.

Warning: During image upload, all the returned fragments must be read from the Host Out FIFO, otherwise the GLINT pipeline will stall. In addition it is strongly recommended that any units which can discard fragments (for instance the following tests: bitmask, alpha, user scissor, screen scissor, stipple, pixel ownership, depth, stencil), are disabled otherwise a shortfall in pixels returned may occur, also leading to deadlock.

Note that because the area of interest in copy/upload/download operations is defined by the rasterizer, it is not limited to rectangular regions.

Color formatting can be used when performing image copies, uploads and downloads. This allows data to be formatted from, or to, any of the supported GLINT color formats.

Rasterizer Mode

A number of long-term modes can be set using the RasterizerMode register, these are:

•Mirror BitMask: This is a single bit flag which specifies the direction bits are checked in the BitMask register. If the bit is reset, the direction is from least significant to most significant (bit 0 to bit 31 ), if the bit is set, it is from most significant to least significant (from bit 31 to bit 0).

•Invert BitMask: This is a single bit which controls the sense of the accept/reject test when using a Bitmask. If the bit is reset then when the BitMask bit is set the fragment is accepted and when it is reset the fragment is rejected. When the bit is set the sense of the test is reversed.

•Fraction Adjust: These 2 bits control the action taken by the rasterizer on receiving a ContinueNewLine command. As GLINT uses a DDA algorithm to render lines, an error accumulates in the DDA value. GLINT provides for greater control of the error by doing one of the following:

•leaving the DDA running, which means errors will be propagated along a line.

•or setting the fraction bits to either zero, a half or almost a half (0×7 FFF).

•Bias Coordinates: Only the integer portion of the values in the DDAs are used to generate fragment addresses. Often the actual action required is a rounding of values, this can be achieved by setting the bias coordinate bit to true which will automatically add almost a half (0×7 FFF) to all input coordinates.

Rasterizer Unit Registers

Real coordinates with fractional parts are provided to the rasterizer in 2's complement 16 bit integer, 16 bit fraction format. The following Table lists the command registers which control the rasterizer unit:

    ______________________________________                                         Register Name                                                                              Description                                                        ______________________________________                                         Render      Starts the rasterization process                                   ContinueNewDom                                                                             Allows the rasterization to continue with a                                    new dominant edge. The dominant edge                                           DDA is reloaded with the new parameters.                                       The subordinate edge is carried on from the                                    previous trapezoid. This allows any convex                                     polygon to be broken down into a collection                                    of trapezoids, with continuity maintained                                      across boundaries. The data field holds the                                    number of scanlines (or sub scanlines) to fill.                                Note this count does not get loaded into the                                   Count register.                                                    ContinueNewSub                                                                             Allows the rasterization to continue with a                                    new subordinate edge. The subordinate                                          DDA is reloaded with the new parameters.                                       The dominant edge is carried on from the                                       previous trapezoid. This is useful when                                        scan converting triangles with a `knee` (i.e.                                  two subordinate edges). The data field holds                                   the number of scanlines (or sub scanlines) to                                  fill. Note this count does not get loaded into                                 the Count register.                                                Continue    Allows the rasterization to continue after                                     new delta value(s) have been loaded, but                                       does not cause either of the trapezoid's                                       edge DDAs to be reloaded. The data field                                       holds the number of scanlines (or sub scan-                                    lines) to fill. Note this count does not get                                   loaded into the Count register.                                    ContinueNewLine                                                                            Allows the rasterization to continue for the                                   next segment in a polyline. The XY position                                    is carried on from the previous line, but the                                  fraction bits in the DDAs can be: kept, set                                    to zero, half, or nearly one half, under                                       control of the Rasterizer-Mode. The data                                       field holds the number of scanlines to fill.                                   Note this count does not get loaded into the                                   Count register. The use of ContinueNew-                                        Line is not recommended for OpenGL                                             because the DDA units will start with a                                        slight error as compared with the value they                                   would have been loaded with for the second                                     and subsequent segments.                                           FlushSpan   Used when antialiasing to force the last span                                  out when not all sub spans may be defined.                         ______________________________________                                    

The following Table shows the control registers of the rasterizer, in the presently preferred embodiment:

    ______________________________________                                         RasterizerMode                                                                           Defines the long term mode of operation of the                                 rasterizer.                                                          StartXDom Initial X value for the dominant edge in                                       trapezoid filling, or initial X value in line draw-                            ing.                                                                 dXDom     Value added when moving from one scanline (or                                  sub scanline) to the next for the dominant edge                                in trapezoid filling. Also holds the change in X                               when plotting lines so for Y major lines this will                             be some fraction (dx/dy), otherwise it is                                      normally ± 1.0, depending on the required                                   scanning direction.                                                  StartXSub Initial X value for the subordinate edge.                            dXSub     Value added when moving from one scanline (or                                  sub scanline) to the next for the subordinate edge                             in trapezoid filling.                                                StartY    Initial scanline (or sub scanline) in trapezoid                                filling, or initial Y position for line drawing.                     dY        Value added to Y to move from one scanline to                                  the next. For X major lines this will be some                                  fraction (dy/dx), otherwise it is normally ± 1.0,                           depending on the required scanning direction.                        Count     Number of pixels in a line. Number of scanlines                                in a trapezoid. Number of sub scanlines in an                                  antialiased trapezoid. Diameter of a point in sub                              scanlines.                                                           BitMaskPattern                                                                           Value used to control the BitMask stipple                                      operation (if enabled).                                              PointTable0                                                                              Antialias point data table. There are 4 words in                     PointTable1                                                                              the table and the register tag is decoded to select                  PointTable2                                                                              a word.                                                              PointTable3                                                                    ______________________________________                                    

For efficiency, the Render command register has a number of bit fields that can be set or cleared per render operation, and which qualify other state information within GLINT. These bits are AreaStippleEnable, LineStippleEnable, ResetLineStipple, TextureEnable FogEnable, CoverageEnable and SubpixelCorrection.

One use of this feature can occur when a window is cleared to a background color. For normal 3D primitives, stippling and fog operations may have been enabled, but these are to be ignored for window clears. Initially the FogMode, AreaStippleMode and LineStippleMode registers are enabled through the UnitEnable bits. Now bits need only be set or cleared within the Render command to achieve the required result, removing the need for the FogMode, AreaStippleMode and LineStippleMode registers to be loaded for every render operation.

The bitfields of the Render command register, in the presently preferred embodiment, are detailed below:

    ______________________________________                                         Bit  Name      Description                                                     ______________________________________                                         0    Area-     This bit, when set, enables area stippling of the                    Stipple-  fragments produced during rasterization. Note                        Enable    that area stipple in the Stipple Unit must be                                  enabled as well for stippling to occur. When                                   this bit is reset no area stippling occurs                                     irrespective of the setting of the area stipple                                enable bit in the Stipple Unit. This bit is useful                             to temporarily force no area stippling for this                                primitive.                                                      1    Line-     This bit, when set, enables line stippling of the                    Stipple-  fragments produced during rasterization in the                       Enable    Stipple Unit. Note that line stipple in the                                    Stipple Unit must be enabled as well for                                       stippling to occur. When this bit is reset no                                  line stippling occurs irrespective of the setting                              of the line stipple enable bit in the Stipple                                  Unit. This bit is useful to temporarily force no                               line stippling for this primitive.                              2    Reset-    This bit, when set, causes the line stipple                          Line-     counters in the Stipple Unit to be reset to                          Stipple   zero, and would typically be used for the first                                segment in a polyline. This action is also                                     qualified by the LineStippleEnable bit and also                                the stipple enable bits in the Stipple Unit.                                   When this bit is reset the stipple counters                                    carry on from where they left off (if line                                     stippling is enabled)                                           3    FastFill- This bit, when set, causes fast block filling of                     Enable    primitives. When this bit is reset the normal                                  rasterization process occurs.                                   4, 5 Fast-Fill-                                                                               This two bit field selects the block size the                        Increment framebuffer supports. The sizes supported and                                  the corresponding codes are:                                                   0 = 8 pixels                                                                   1 = 16 pixels                                                                  2 = 32 pixels                                                   6, 7 Primitive-                                                                               This two bit field selects the primitive type to                     Type      rasterize. The primitives are:                                                 0 = Line                                                                       1 = Trapezoid                                                                  2 = Point                                                       8    Antialias-                                                                               This bit, when set, causes the generation of sub                     Enable    scanline data and the coverage value to be                                     calculated for each fragment. The number of                                    sub pixel samples to use is controlled by the                                  AntialiasingQuality bit. When this bit is reset                                normal rasterization occurs.                                    9    Antialias-                                                                               This bit, when set, sets the sub pixel resolution                    ing Quality                                                                              to be 8 × 8                                                              When this bit is reset the sub pixel resolution                                is 4 × 4.                                                 10   UsePoint- When this bit and the AntialiasingEnable are                         Table     set, the dx values used to move from one                                       scanline to the next are derived from the Point                                Table.                                                          11   SyncOn-   This bit, when set, causes a number of actions:                      BitMask   The least significant bit or most significant bit                              (depending on the MirrorBitMask bit) in the                                    Bit Mask register is extracted and optionally                                  inverted (controlled by the InvertBitMask                                      bit). If this bit is 0 then the corresponding                                  fragment is culled from being drawn. After                                     every fragment the Bit Mask register is                                        rotated by one bit. If all the bits in the                                     Bit Mask register have been used then                                          rasterization is suspended until a new                                         BitMaskPattern is received. If any                                             other register is written while the rasterization                              is suspended then the rasterization is aborted.                                The register write which caused the abort is                                   then processed as normal. Note the behavior is                                 slightly different when the SyncOnHostData                                     bit is set to prevent a deadlock from                                          occurring. In this case the rasterization doesn't                              suspend when all the bits have been used and                                   if new BitMaskPattern data words are not                                       received in a timely manner then the sub-                                      sequent fragments will just reuse the bitmask.                  12   SyncOn-   When this bit is set a fragment is produced                          HostData  only when one of the following registers has                                   been written by the host: Depth, FBColor,                                      Stencil or Color. If SyncOnBitMask is reset,                                   then if any register other than one of these                                   four is written to, the rasterization is aborted.                              If SyncOnBitMask is set, then if any register                                  other than one of these four, or BitMask-                                      Pattern, is written to, the rasterization is                                   aborted. The register write which caused the                                   abort is then processed as normal. Writing to                                  the BitMaskPattern register doesn't cause any                                  fragments to be generated, but just updates the                                BitMask register.                                               13   Texture-  This bit, when set, enables texturing of the                         Enable    fragments produced during rasterization. Note                                  that the Texture Units must be suitably                                        enabled as well for any texturing to occur.                                    When this bit is reset no texturing occurs                                     irrespective of the setting of the Texture Unit                                controls. This bit is useful to temporarily force                              no texturing for this primitive.                                14   Fog-      This bit, when set, enables fogging of the                           Enable    fragments produced during rasterization. Note                                  that the Fog Unit must be suitably enabled as                                  well for any fogging to occur. When this bit is                                reset no fogging occurs irrespective of the                                    setting of the Fog Unit controls. This bit is                                  useful to temporarily force no fogging for this                                primitive.                                                      15   Coverage- This bit, when set, enables the coverage value                       Enable    produced as part of the antialiasing to weight                                 the alpha value in the alpha test unit. Note                                   that this unit must be suitably enabled as well.                               When this bit is reset no coverage application                                 occurs irrespective of the setting of the                                      AntialiasMode in the Alpha Test unit.                           16   SubPixel- This bit, when set enables the sub pixel                             Correction                                                                               correction of the color, depth, fog and texture                      Enable    values at the start of a scanline. When this bit                               is reset no correction is done at the start of a                               scanline. Sub pixel corrections are only                                       applied to aliased trapezoids.                                  ______________________________________                                    

A number of long-term rasterizer modes are stored in the RasterizerMode register as shown below:

    ______________________________________                                         Bit  Name        Description                                                   ______________________________________                                         0    MirrorBit-  When this bit is set the bitmask bits are                          Mask        consumed from the most significant end                                         towards the least significant end. When                                        this bit is reset the bitmask bits are con-                                    sumed from the least significant end                                           towards the most significant end.                             1    InvertBit-  When this bit is set the bitmask is inverted                       Mask        first before being tested.                                    2, 3 Fraction-   These bits control the action of a                                 Adjust      ContinueNewLine command and specify                                            how the fraction bits in the Y and XDom                                        DDAs are adjusted                                                              0: No adjustment is done                                                       1: Set the fraction bits to zero                                               2: Set the fraction bits to half                                               3: Set the fraction to nearly half, i.e. 0x7fff               4, 5 BiasCoordinates                                                                            These bits control how much is added                                           onto the StartXDom, StartXSub and                                              StartY values when they are loaded into                                        the DDA units. The original registers are                                      not affected:                                                                  0: Zero is added                                                               1: Half is added                                                               2: Nearly half, i.e. 0x7fff is added                          ______________________________________                                    

Scissor Unit

Two scissor tests are provided in GLINT, the User Scissor test and the Screen Scissor test. The user scissor checks each fragment against a user supplied scissor region; the screen scissor checks that the fragment lies within the screen.

This test may reject fragments if some part of a window has been moved off the screen. It will not reject fragments if part of a window is simply overlapped by another window (GID testing can be used to detect this).

Stipple Unit

Stippling is a process whereby each fragment is checked against a bit in a defined pattern, and is rejected or accepted depending on the result of the stipple test. If it is rejected it undergoes no further processing; otherwise it proceeds down the pipeline. GLINT supports two types of stippling, line and area.

Area Stippling

A 32×32 bit area stipple pattern can be applied to fragments. The least significant n bits of the fragment's (X,Y) coordinates, index into a 2D stipple pattern. If the selected bit in the pattern is set, then the fragment passes the test, otherwise it is rejected. The number of address bits used, allow regions of 1,2,4,8,16 and 32 pixels to be stippled. The address selection can be controlled independently in the X and Y directions. In addition the bit pattern can be inverted or mirrored. Inverting the bit pattern has the effect of changing the sense of the accept/reject test. If the mirror bit is set the most significant bit of the pattern is towards the left of the window, the default is the converse.

In some situations window relative stippling is required but coordinates are only available screen relative. To allow window relative stippling, an offset is available which is added to the coordinates before indexing the stipple table. X and Y offsets can be controlled independently.

Line Stippling

In this test, fragments are conditionally rejected on the outcome of testing a linear stipple mask. If the bit is zero then the test fails, otherwise it passes. The line stipple pattern is 16 bits in length and is scaled by a repeat factor r (in the range 1 to 512). The stipple mask bit b which controls the acceptance or rejection of a fragment is determined using:

    b=(floor (s/r)) mod 16

where s is the stipple counter which is incremented for every fragment (normally along the line). This counter may be reset at the start of a polyline, but between segments it continues as if there were no break.

The stipple pattern can be optionally mirrored, that is the bit pattern is traversed from most significant to least significant bits, rather than the default, from least significant to most significant.

Color DDA Unit

The color DDA unit is used to associate a color with a fragment produced by the rasterizer. This unit should be enabled for rendering operations and disabled for pixel rectangle operations (i.e. copies, uploads and downloads). Two color modes are supported by GLINT, true color RGBA and color index (CI).

Gouraud Shading

When in Gouraud shading mode, the color DDA unit performs linear interpolation given a set of start and increment values. Clamping is used to ensure that the interpolated value does not underflow or overflow the permitted color range.

For a Gouraud shaded trapezoid, GLINT interpolates from the dominant edge of a trapezoid to the subordinate edges. This means that two increment values are required per color component, one to move along the dominant edge and one to move across the span to the subordinate edge.

Note that if one is rendering to multiple buffers and has initialized the start and increment values in the color DDA unit, then any subsequent Render command will cause the start values to be reloaded.

If subpixel correction has been enabled for a primitive, then any correction required will be applied to the color components.

Flat Shading

In flat shading mode, a constant color is associated with each fragment. This color is loaded into the ConstantColor register.

Texture Unit

The texture unit combines the incoming fragment's color (generated in the color DDA unit) with a value derived from interpolating texture map values (texels).

Texture application consists of two stages; derivation of the texture color from the texels (a filtering process) and then application of the texture color to the fragment's color, which is dependent on the application mode (Decal, Blend or Modulate).

GLINT 300SX compared with the GLINT 300TX

Both the GLINT 300SX and GLINT 300TX support all the filtering and application modes described in this section. However, when using the GLINT 300SX, texel values, interpolants and texture filter selections are supplied by the host. This implies that texture coordinate interpolation and texel extraction are performed by the host using texture maps resident on the host. The recommended technique for performing texture mapping using the GLINT 300SX is to scan convert primitives on the host and render fragments as GLINT point primitives.

The GLINT 300TX automatically generates all data required for texture application as textures are stored in the localbuffer and texture parameter interpolation with full perspective correction takes place within the processor. Thus the GLINT 300TX is the processor of choice when full texture mapping acceleration is desired, the GLINT 300SX is more suitable in applications where the performance of texture mapping is not critical.

Texture Color Generation.

Texture color generation supports all the filter modes of OpenGL, that is:

Minification:

Nearest

Linear

NearestMipMapNearest

NearestMipMapLinear

LinearMipMapNearest

LinearMipMapLinear

Magnification:

Nearest

Linear

Minification is the name given to the filtering process used whereby multiple texels map to a fragment, while magnification is the name given to the filtering process whereby only a portion of a single texel maps to a single fragment.

Nearest is the simplest form of texture mapping where the nearest texel to the sample location is selected with no filtering applied.

Linear is a more sophisticated algorithm which is dependent on the type of primitive. For lines (which are 1D), it involves linear interpolation between the two nearest texels, for polygons and points which are considered to have finite area, linear is in fact bi-linear interpolation which interpolates between the nearest 4 texels.

Mip Mapping is a technique to allow the efficient filtering of texture maps when the projected area of the fragment covers more than one texel (ie. minification). A hierarchy of texture maps is held with each one being half the size (or one quarter the area) of the preceding one. A pair of maps are selected, based on the projected area of the texture. In terms of filtering this means that three filter operations are performed: one on the first map, one on the second map and one between the maps. The first filter name (Nearest or Linear) in the MipMap name specifies the filtering to do on the two maps, and the second filter name specifies the filtering to do between maps. So for instance, linear mapping between two maps, with linear interpolation between the results is supported (LinearMipMapLinear), but linear interpolation on one map, nearest on the other map, and linear interpolation between the two is not supported.

The filtering process takes a number of texels and interpolants, and with the current texture filter mode produces a texture color.

Fog Unit

The fog unit is used to blend the incoming fragment's color (generated by the color DDA unit, and potentially modified by the texture unit) with a predefined fog color. Fogging can be used to simulate atmospheric fogging, and also to depth cue images.

Fog application has two stages; derivation of the fog index for a fragment, and application of the fogging effect. The fog index is a value which is interpolated over the primitive using a DDA in the same way color and depth are interpolated. The fogging effect is applied to each fragment using one of the equations described below.

Note that although the fog values are linearly interpolated over a primitive the fog values can be calculated on the host using a linear fog function (typically for simple fog effects and depth cuing) or a more complex function to model atmospheric attenuation. This would typically be an exponential function.

Fog Index Calculation--The Fog DDA

The fog DDA is used to interpolate the fog index (f) across a primitive. The mechanics are similar to those of the other DDA units, and horizontal scanning proceeds from dominant to subordinate edge as discussed above.

The DDA has an internal range of approximately +511 to -512, so in some cases primitives may exceed these bounds. This problem typically occurs for very large polygons which span the whole depth of a scene. The correct solution is to tessellate the polygon until polygons lie within the acceptable range, but the visual effect is frequently negligible and can often be ignored.

The fog DDA calculates a fog index value which is clamped to lie in the range 0.0 to 1.0 before it is used in the appropriate fogging equation. (Fogging is applied differently depending on the color mode.)

Antialias Application Unit

Antialias application controls the combining of the coverage value generated by the rasterizer with the color generated in the color DDA units. The application depends on the color mode, either RGBA or Color Index (CI).

Antialias Application

When antialiasing is enabled this unit is used to combine the coverage value calculated for each fragment with the fragment's alpha value. In RGBA mode the alpha value is multiplied by the coverage value calculated in the rasterizer (its range is 0% to 100% ). The RGB values remain unchanged and these are modified later in the Alpha Blend unit which must be set up appropriately. In CI mode the coverage value is placed in the lower 4 bits of the color field. The Color Look Up Table is assumed to be set up such that each color has 16 intensities associated with it, one per coverage entry.

Polygon Antialiasing

When using GLINT to render antialiased polygons, depth buffering cannot be used. This is because the order the fragments are combined in is critical in producing the correct final color. Polygons should therefore be depth sorted, and rendered front to back, using the alpha blend modes: SourceAlphaSaturate for the source blend function and One for the destination blend function. In this way the alpha component of a fragment represents the percentage pixel coverage, and the blend function accumulates coverage until the value in the alpha buffer equals one, at which point no further contributions can made to a pixel.

For the antialiasing of general scenes, with no restrictions on rendering order, the accumulation buffer is the preferred choice. This is indirectly supported by GLINT via image uploading and downloading, with the accumulation buffer residing on the host.

When antialiasing, interpolated parameters which are sampled within a fragment (color, fog and texture), will sometimes be unrepresentative of a continuous sampling of a surface, and care should be taken when rendering smooth shaded antialiased primitives. This problem does not occur in aliased rendering, as the sample point is consistently at the center of a pixel.

Alpha Test Unit

The alpha test compares a fragment's alpha value with a reference value. Alpha testing is not available in color index (CI) mode. The alpha test conditionally rejects a fragment based on the comparison between a reference alpha value and one associated with the fragment.

Localbuffer Read/Write Unit

The localbuffer holds the Graphic ID, FrameCount, Stencil and Depth data associated with a fragment. The localbuffer read/write unit controls the operation of GID testing, depth testing and stencil testing.

Localbuffer Read

The LBReadMode register can be configured to make 0, 1 or 2 reads of the localbuffer. The following are the most common modes of access to the localbuffer:

•Normal rendering without depth, stencil or GID testing. This requires no localbuffer reads or writes.

•Normal rendering without depth or stencil testing and with GID testing. This requires a localbuffer read to get the GID from the localbuffer.

•Normal rendering with depth and/or stencil testing required which conditionally requires the localbuffer to be updated. This requires localbuffer reads and writes to be enabled.

•Copy operations. Operations which copy all or part of the localbuffer with or without GID testing. This requires reads and writes enabled.

•Image upload/download operations. Operations which download depth or stencil information to the local buffer or read depth, stencil fast clear or GID from the localbuffer.

Localbuffer Write

Writes to the localbuffer must be enabled to allow any update of the localbuffer to take place. The LBWriteMode register is a single bit flag which controls updating of the buffer.

Pixel Ownership (GID) Test Unit

Any fragment generated by the rasterizer may undergo a pixel ownership test. This test establishes the current fragment's write permission to the localbuffer and framebuffer.

Pixel Ownership Test

The ownership of a pixel is established by testing the GID of the current window against the GID of a fragment's destination in the GID buffer. If the test passes, then a write can take place, otherwise the write is discarded. The sense of the test can be set to one of: always pass, always fail, pass if equal, or pass if not equal. Pass if equal is the normal mode. In GLINT the GID planes, if present, are 4 bits deep allowing 16 possible Graphic ID's. The current GID is established by setting the Window register.

If the unit is disabled fragments pass through undisturbed.

Stencil Test Unit

The stencil test conditionally rejects fragments based on the outcome of a comparison between the value in the stencil buffer and a reference value. The stencil buffer is updated according to the current stencil update mode which depends on the result of the stencil test and the depth test.

Stencil Test

This test only occurs if all the preceding tests (bitmask, scissor, stipple, alpha, pixel ownership) have passed. The stencil test is controlled by the stencil function and the stencil operation. The stencil function controls the test between the reference stencil value and the value held in the stencil buffer. The stencil operation controls the updating of the stencil buffer, and is dependent on the result of the stencil and depth tests.

If the stencil test is enabled then the stencil buffer will be updated depending on the outcome of both the stencil and the depth tests (if the depth test is not enabled the depth result is set to pass).

In addition a comparison bit mask is supplied in the StencilData register. This is used to establish which bits of the source and reference value are used in the stencil function test. In addition it should normally be set to exclude the top four bits when the stencil width has been set to 4 bits in the StencilMode register.

The source stencil value can be from a number of places as controlled by a field in the StencilMode register:

    ______________________________________                                         LBWriteData Stencil                                                                         Use                                                               ______________________________________                                         Test logic   This is the normal mode.                                          Stencil register                                                                            This is used, for instance, in the OpenGL                                      draw pixels function where the host                                            supplies the stencil values in the Stencil                                     register. This is used when a constant                                         stencil value is needed, for example, when                                     clearing the stencil buffer when fast clear                                    planes are not available.                                         LBSourceData:                                                                               This is used, for instance, in the OpenGL                         (stencil value read                                                                         copy pixels function when the stencil                             from the localbuffer)                                                                       planes are to be copied to the destination.                                    The source is offset from the destination                                      by the value in LBSourceOffset register.                          Source stencil value                                                                        This is used, for instance, in the OpenGL                         read from the local-                                                                        copy pixels function when the stencil                             buffer       planes in the destination are not to be                                        updated. The stencil data will come either                                     from the localbuffer date, or the                                              FCStencil register, depending on whether                                       fast clear operations are enabled.                                ______________________________________                                    

Depth Test Unit

The depth (Z) test, if enabled, compares a fragment's depth against the corresponding depth in the depth buffer. The result of the depth test can effect the updating of the stencil buffer if stencil testing is enabled. This test is only performed if all the preceding tests (bitmask, scissor, stipple, alpha, pixel ownership, stencil) have passed. The source value can be obtained from a number of places as controlled by a field in the DepthMode register:

    ______________________________________                                         Source     Use                                                                 ______________________________________                                         DDA (see below)                                                                           This is used for normal Depth buffered 3D                                      rendering.                                                          Depth register                                                                            This is used, for instance, in the OpenGL                                      draw pixels function where the host                                            supplies the depth values through the                                          Depth register. Alternatively this is used                                     when a constant depth value is needed,                                         for example, when clearing the depth                                           buffer (when fast clear planes are not                                         available) or 2D rendering where the                                           depth is held constant.                                             LBSourceData:                                                                             This is used, for instance, in the OpenGL                           Source depth                                                                              copy pixels function when the depth                                 value from the                                                                            planes are to be copied to the destination.                         localbuffer                                                                    Source Depth                                                                              This is used, for instance, in the OpenGL                                      copy pixels function when the depth                                            planes in the destination are not updated.                                     The depth data will come either from the                                       localbuffer or the FCDepth register                                            depending the state of the Fast Clear                                          modes in operation.                                                 ______________________________________                                    

When using the depth DDA for normal depth buffered rendering operations the depth values required are similar to those required for the color values in the color DDA unit:

ZStart=Start Z Value

dZdYDom=Increment along dominant edge.

dZdX=Increment along the scan line.

The dZdX value is not required for Z-buffered lines.

The depth unit must be enabled to update the depth buffer. If it is disabled then the depth buffer will only be updated if ForceLBUpdate is set in the Window register.

Framebuffer Read/Write Unit

Before rendering can take place GLINT must be configured to perform the correct framebuffer read and write operations. Framebuffer read and write modes effect the operation of alpha blending, logic ops, write masks, image upload/download operations and the updating of pixels in the framebuffer.

Framebuffer Read

The FBReadMode register allows GLINT to be configured to make 0, 1 or 2 reads of the framebuffer. The following are the most common modes of access to the framebuffer: Note that avoiding unnecessary additional reads will enhance performance.

•Rendering operations with no logical operations, software writemasking or alpha blending. In this case no read of the framebuffer is required and framebuffer writes should be enabled.

•Rendering operations which use logical ops, software write masks or alpha blending. In these cases the destination pixel must be read from the framebuffer and framebuffer writes must be enabled.

•Image copy operations. Here setup varies depending on whether hardware or software write masks are used. For software write masks, the framebuffer needs two reads, one for the source and one for the destination. When hardware write masks are used (or when the software write mask allows updating of all bits in a pixel) then only one read is required.

•Image upload. This requires reading of the destination framebuffer reads to be enabled and framebuffer writes to be disabled.

•Image download. In this case no framebuffer read is required (as long as software writemasking and logic ops are disabled) and the write must be enabled.

For both the read and the write operations, an offset is added to the calculated address. The source offset (FBSourceOffset) is used for copy operations. The pixel offset (FBPixelOffset) can be used to allow multi-buffer updates. The offsets should be set to zero for normal rendering.

The data read from the framebuffer may be tagged either FBDefault (data which may be written back into the framebuffer or used in some manner to modify the fragment color) or FBColor (dam which will be uploaded to the host). The table below summarizes the framebuffer read/write control for common rendering operations:

    ______________________________________                                         Read-  Read-              Read Data                                                                              Rendering                                    Source Destination                                                                              Writes   Type    Operation                                    ______________________________________                                         Disabled                                                                              Disabled  Enabled  --      Rendering with                                                                 no logical                                                                     operations,                                                                    software write                                                                 masks or blend-                                                                ing.                                         Disabled                                                                              Disabled  Enabled  --      Image download.                              Disabled                                                                              Enabled   Disabled FBColor Image upload.                                Enabled                                                                               Disabled  Enabled  FBDefault                                                                              Image copy with                                                                hardware write                                                                 masks.                                       Disabled                                                                              Enabled   Enabled  FBDefault                                                                              Rendering using                                                                logical operations,                                                            software write                                                                 masks or                                                                       blending.                                    Enabled                                                                               Enabled   Enabled  FBDefault                                                                              Image copy with                                                                software write-                                                                masks.                                       ______________________________________                                    

Framebuffer Write

Framebuffer writes must be enabled to allow the framebuffer to be updated. A single 1 bit flag controls this operation.

The framebuffer write unit is also used to control the operation of fast block fills, if supported by the framebuffer. Fast fill rendering is enabled via the FastFillEnable bit in the Render command register, the framebuffer fast block size must be configured to the same value as the FastFillIncrement in the Render command register. The FBBlockColor register holds the data written to the framebuffer during a block fill operation and should be formatted to the `raw` framebuffer format. When using the framebuffer in 8 bit packed mode the data should be replicated into each byte. When using the framebuffer in packed 16 bit mode the data should be replicated into the top 16 bits.

When uploading images the UpLoadData bit can be set to allow color formatting (which takes place in the Alpha Blend unit).

It should be noted that the block write capability provided by the chip of the presently preferred embodiment is itself believed to be novel. According to this new approach, a graphics system can do masked block writes of variable length (e.g. 8, 16, or 32 pixels, in the presently preferred embodiment). The rasterizer defines the limits of the block to be written, and hardware masking logic in the frame-buffer interface permits the block to be filled in, with a specified primitive, only up to the limits of the object being rendered. Thus the rasterizer can step by the Block Fill increment. This permits the block-write capabilities of the VRAM chips to be used optimally, to minimize the length which must be written by separate writes per pixel.

Alpha Blend Unit

Alpha blending combines a fragment's color with those of the corresponding pixel in the framebuffer. Blending is supported in RGBA mode only.

Alpha Blending

The alpha blend unit combines the fragment's color value with that stored in the framebuffer, using the blend equation:

    C.sub.o =C.sub.s S+C.sub.d D

where: C_(o) is the output color; C_(s) is the source color (calculated internally); C_(d) is the destination color read from the framebuffer; S is the source blending weight; and D is the destination blending weight. S and D are not limited to linear combinations; lookup functions can be used to implement other combining relations.

If the blend operations require any destination color components then the framebuffer read mode must be set appropriately.

Image Formatting

The alpha blend and color formatting units can be used to format image dam into any of the supported GLINT framebuffer formats.

Consider the case where the framebuffer is in RGBA 4:4:4:4 mode, and an area of the screen is to be uploaded and stored in an 8 bit RGB 3:3:2 format. The sequence of operations is:

•Set the rasterizer as appropriate

•Enable framebuffer reads

•Disable framebuffer writes and set the UpLoadData bit in the FBWriteMode register

•Enable the alpha blend unit with a blend function which passes the destination value and ignores the source value (source blend Zero, destination blend One) and set the color mode to RGBA 4:4:4:4

•Set the color formatting unit to format the color of incoming fragments to an 8 bit RGB 3:3:2 framebuffer format.

The upload now proceeds as normal. This technique can be used to upload data in any supported format.

The same technique can be used to download data which is in any supported framebuffer format, in this case the rasterizer is set to sync with FBColor, rather than Color. In this case framebuffer writes are enabled, and the UpLoadData bit cleared.

Color Formatting Unit

The color formatting unit converts from GLINT's internal color representation to a format suitable to be written into the framebuffer. This process may optionally include dithering of the color values for framebuffers with less than 8 bits width per color component. If the unit is disabled then the color is not modified in any way.

As noted above, the framebuffer may be configured to be RGBA or Color Index (CI).

Color Dithering

GLINT uses an ordered dither algorithm to implement color dithering. Several types of dithering can be selected.

If the color formatting unit is disabled, the color components RGBA are not modified and will be truncated when placed in the framebuffer. In CI mode the value is rounded to the nearest integer. In both cases the result is clamped to a maximum value to prevent overflow.

In some situations only screen coordinates are available, but window relative dithering is required. This can be implemented by adding an optional offset to the coordinates before indexing the dither tables. The offset is a two bit number which is supplied for each coordinate, X and Y. The XOffset, YOffset fields in the DitherMode register control this operation, if window relative coordinates are used they should be set to zero.

Logical Op Unit

The logical op unit performs two functions; logic operations between the fragment color (source color) and a value from the framebuffer (destination color); and, optionally, control of a special GLINT mode which allows high performance flat shaded rendering.

High Speed Flat Shaded Rendering

A special GLINT rendering mode is available which allows high speed rendering of unshaded images. To use the mode the following constraints must be satisfied:

•Flat shaded aliased primitive

•No dithering required

•No logical ops

•No stencil, depth or GID testing required

•No alpha blending

The following are available:

•Bit masking in the rasterizer

•Area and line stippling

•User and Screen Scissor test

If all the conditions are met then high speed rendering can be achieved by setting the FBWriteData register to hold the framebuffer data (formatted appropriately for the framebuffer in use) and setting the UseConstantFBWriteData bit in the LogicalOpMode register. All unused units should be disabled.

This mode is most useful for 2D applications or for clearing the framebuffer when the memory does not support block writes. Note that FBWriteData register should be considered volatile when context switching.

Logical Operations

The logical operations supported by GLINT are:

    ______________________________________                                         Mode  Name     Operation Mode  Name    Operation                               ______________________________________                                         0     Clear    0          8    Nor     ˜(S | D)                 1     And      S & D      9    Equivalent                                                                             ˜(S   D)                          2     And      S & ˜D                                                                             10    Invert  ˜D                                      Reverse                                                                  3     Copy     S         11    Or Reverse                                                                             S | ˜D                   4     And      ˜S & D                                                                             12    Copy    ˜S                                      Inverted                 Invert                                          5     Noop     D         13    Or Invert                                                                              ˜S | D                   6     Xor      S   D     14    Nand    ˜(S & D)                          7     Or       S | D                                                                           15    Set     1                                       ______________________________________                                    

Where:

S=Source (fragment) Color, D=Destination (framebuffer) Color.

For correct operation of this unit in a mode which takes the destination color, GLINT must be configured to allow reads from the framebuffer using the FBReadMode register.

GLINT makes no distinction between RGBA and CI modes when performing logical operations. However, logical operations are generally only used in CI mode.

Framebuffer Write Masks

Two types of framebuffer write masking are supported by GLINT, software and hardware. Software write masking requires a read from the framebuffer to combine the fragment color with the framebuffer color, before checking the bits in the mask to see which planes are writeable. Hardware write masking is implemented using VRAM write masks and no framebuffer read is required.

Software Write Masks

Software write masking is controlled by the FBSoftwareWriteMask register. The data field has one bit per framebuffer bit which when set, allows the corresponding framebuffer bit to be updated. When reset it disables writing to that bit. Software write masking is applied to all fragments and is not controlled by an enable/disable bit. However it may effectively be disabled by setting the mask to all 1's. Note that the ReadDestination bit must be enabled in the FBReadMode register when using software write masks, in which some of the bits are zero.

Hardware Write Masks

Hardware write masks, if available, are controlled using the FBHardwareWriteMask register. If the framebuffer supports hardware write masks, and they are to be used, then software write masking should be disabled (by setting all the bits in the FBSoftwareWriteMask register). This will result in fewer framebuffer reads when no logical operations or alpha blending is needed.

If the framebuffer is used in 8 bit packed mode, then an 8 bit hardware write mask must be replicated to all 4 bytes of the FBHardwareWriteMask register. If the framebuffer is in 16 bit packed mode then the 16 bit hardware write mask must be replicated to both halves of the FBHardwareWriteMask register.

Host Out Unit

Host Out Unit controls which registers are available at the output FIFO, gathering statistics about the rendering operations (picking and extent testing) and the synchronization of GLINT via the Sync register. These three functions are as follows:

•Message filtering. This unit is the last unit in the core so any message not consumed by a preceding unit will end up here. These messages will fall in to three classifications: Rasterizer messages which are never consumed by the earlier units, messages associated with image uploads, and finally programmer mistakes where an invalid message was written to the input FIFO. Synchronization messages are a special category and are dealt with later. Any messages not filtered out are passed on the output FIFO.

•Statistic Collection. Here the active step messages are used to record the extent of the rectangular region where rasterization has been occurring, or if rasterization has occurred inside a specific rectangular region. These facilities are useful for picking and debug activities.

•Synchronization. It is often useful for the controlling software to find out when some rendering activity has finished, to allow the timely swapping or sharing of buffers, reading back of state, etc. To achieve this the software would send a Sync message and when this reached this unit any preceding messages or their actions are guaranteed to have finished. On receiving the Sync message it is entered into the FIFO and optionally generates an interrupt.

Implementation of Sub-Pixel Calculations

Some further details of the hardware implementation which provides accelerated subpixel calculations will now be described.

The rasterization rules of OpenGL and X both state than butting polygons must only touch pixels along the common boundary once (when not antialiasing). The simplest way to ensure this is never to plot the pixels along the top and right edges of a polygon. The GLINT rasterizer will not generate fragments along the right hand edge, but the host must set up the start and end scanlines as required to miss either the top or bottom edges.

As part of the rasterization process a number of parameters (color, depth, fog and texture) are calculated for each fragment generated. These are calculated in DDA unit down stream under the guidance of the rasterizer step messages. The ideal way to calculate these values is to use the fragments XY coordinate and substitute this into the plane equation for each parameter in turn. This technique gives the best result, however it is computationally expensive so it is normal to use an incremental method such as a DDA to approximate to it. The DDA method introduces some errors of its own:

•An incremental error due to the finite precision of the delta values. To overcome this source of errors enough fractional bits are used so that the error cannot propagate into the actual bit range of the DDA where the parameter value is extracted from.

•The start value for a parameter, P, can be nearly dPdx (one step in the X direction) out because the value calculated as a result of a Y step (shown as a circle in FIG. 5) corresponds to the value of the sample on the edge and not at the center of the first fragment to be drawn. It is necessary to correct for this error to eliminate bright edge artifacts and achieve high quality rendering.

This correction is needed for every scanline. A similar correction is needed at the start of the primitive because the parameter value at the start vertex is unlikely to lie on the horizontal center of a pixel so need adjustment in Y. In the presently preferred embodiment this correction is handled by software. (However, it is contemplated that in future versions it would be advantageous to have the chip handle this function as well. The same multiplexing method as described above can be used, but with the dyDom values feeding into another 4 input multiplexer, sequenced as described above.)

If dErr is the distance the edge is away from the pixel's center (must be <1) and dPdx is the change in P for unit change in x then the correct value at the first sample point is:

    Px=Py+dErr×dPdx

FIG. 5 illustrates this.

Sub-Pixel Calculations in the Rasterizer

The distance dErr is sent by the rasterizer in the SubPixelCorrection message and whenever this message is received by a DDA unit the above equation is implemented (in an incremental form to avoid the multiply operation). The correction dErr is sent in sign magnitude format with 4 bits of magnitude and a sign.

Subpixel correction must be enabled by the SubPixelCorrectionEnable bit in the Render message if it is required.

Antialiasing presents a much more complex problem to solve in that the sample point for the parameters must be inside the boundary of the fragment, but this may not be the center of the pixel anymore. Near horizontal edges can give rise to a dErr value which approaches the width of the screen (or window). Two methods can be used to overcome this:

•The sample point can be moved to be within the boundary by `micro nudging` the DDAs in X and Y.

•The parameter being interpolated can be integrated over the interior sub pixel sample points and then divided by the number of interior points (this is the method in the OpenGL spec).

In both these cases the changes to the DDA units are too extensive given the other problems this method of antialiasing has (the coverage calculation doesn't take into account sub pixel visibility and doesn't work well with a depth buffer). No subpixel corrections are done for antialiased primitives.

A triangle has three edges: a dominant edge and two subordinate edges. The dominant edge is the edge which covers the maximum Y extent (as illustrated in FIG. 4A). The chip of the presently preferred embodiment provides the capability to always scan convert from the dominant edge; this is preferable, and will be assumed in the following description.

The triangle is further decomposed (if it has a knee) into two triangles with flat tops or bottoms to facilitate the edge walking stage.

The sequence of actions the host needs to do to render a triangle (with a `knee`) are:

•Load up the edge parameters and derivatives for the dominant edge and the first subordinate edges in the first triangle.

•The Render message starts the scan conversion of the first triangle, working from the dominant edge. This means that for triangles where the knee is on the left we are scanning right to left, and for triangles where the knee is on the right we are scanning more conventionally left to right.

•Load up the edge parameters and derivatives for the remaining subordinate edge in the second triangle.

•The ContinueNewSub message starts the scan conversion of the second triangle.

The order the fragments are produced in is dependent on the parameters. If the origin is in the bottom left corner and the Y derivative is both positive then the scan conversion proceeds from the bottom towards the top. If the derivative is negative then scan conversion proceeds in the opposite direction. The X direction is always from the dominant edge. For a screen aligned rectangle both vertical edges are equally dominant so you can choose which edge to render from so effecting the scanning direction.

Sub-Pixel Calculations in the DDA Units

The Color DDA will be used to illustrate the mechanics of the sub pixel correction process, but it should be noted that the same comments, with obvious extensions to the different resolutions and dynamic ranges apply to all the DDA units which may be present (e.g. depth, fog, texture S, T and Q). As noted above, the Color DDA unit is responsible for generating the color information (red, green, blue and alpha) components under the direction of the Step messages from the Rasterizer Unit. Subpixel corrections can be applied to correct for an initial start error on a span.

All four components are always generated even though in Color Index mode only one is needed. The color index value is assumed to be in the least significant byte of the Color message by units downstream.

The output of this unit is the Color message, which is normally generated on an active step. In normal operation this causes two messages per active fragment in the message stream.

The block diagram of one of the component DDAs (where C represents R, G, B or A) is shown in FIG. 6. Some notes on this Figure are:

•When the shading mode is Flat the color value is taken from the ConstantColor register and not from the DDA unit. This register holds the 32 bit RGBA value (R is in the least significant byte) or the raw 32 bit framebuffer data depending on how the down stream units are set up.

•The adder/subtracter (labelled "+/-") normally works as an adder except when subpixel compensation is being applied where it can work in either mode depending on the direction of the correction. At first sight an additive correction is all that is necessary because we always correct in the direction that the dx derivative is set up. However the geometry of the primitive has been biased by nearly a half (of a pixel) to make the rasterization rules simpler to implement, while the interpolated parameters are set up with respect to the unbiased geometry.

•The nomenclature in this Figure is as follows:

    ______________________________________                                         dCdx    change in component C for unit change in X                             dCdyDom change in component C for unit change in Y along                               the dominant edge of a trapezoid, or along a line                      CStart  start value of component C                                             ______________________________________                                    

•The Cx register holds the color component as a span is filled. The Cy register holds the value of the color component as we step up the dominant edge of the primitive and is used to seed the span interpolation.

•The start and derivative number format is 2's complement fixed point integer: 9 bits integer and 15 bits fraction. The internal format has an additional 2 bits to extend the range to give more protection against over or underflow. This guard band is only designed to cope with small amounts of overflow (or underflow) when one or two extra deltas are added during rasterization, usually as a result of the sample point (i.e. pixel center) being outside of the boundary of the primitive. If the additional range provided by the guard band is exceeded then wrap around will occur and an abrupt change in the color will be visible (the clamp logic cannot prevent this). This situation will only occur if the delta values are set up incorrectly for the number of iterations the DDA works through, i.e. the gradient is specified as being steeper than it really is.

•The Clamp logic prevents the interpolated value wrapping around (when the color component is extracted) when overflowing or underflowing occurs during interpolation.

This should not be necessary if the derivatives, etc. are set up correctly and sub pixel corrections are done at the start of every span. However we are working with finite precision numbers so slight rounding errors will occur.

The clamp logic is simply a multiplexer with selects either the result, zero or 255.0 based on comparing the result against 255.0 and the sign bit.

•Multiplexer C is used to implement the sub pixel correction. The need for the sub pixel correction is shown in FIG. 5.

The start value has already been compensated to move it to the center of a pixel in the Y directions. This only needs to be done once per triangle and is done in software. As the DDA is stepped in Y along the dominant edge the value of the interpolated parameter is calculated at the points marked by circles, whereas the value is required in the pixel's center.

The distance dErr is sent by the rasterizer in the SubPixelCorrection message and whenever this message is received the above equation is implemented. The SubPixelCorrection message allows for 4 bits of resolution of dErr. The correction is applied to a resolution of 1/16 of the dx derivative. The multiply operation is too expensive to do using a multiplier so it is implemented by addition of the partial products. For a positive correction this means adding in dCdx/2 when bit 3 is set, dCdx/4 when bit 2 is set, dCdx/8 when bit 1 is set and dCdx/16 when bit 0 is set. For a negative correction the corresponding partial products are subtracted.

The division of dCdx is always done by a power of two so is just an arithmetic right shift (to retain the sign of the dCdx).

The correction will add one cycle for every bit set in dErr so worst case will take 4 cycles per scanline, but on average 2 cycles. This time is hidden under the page break time allowed for every scanline so will not impact performance.

FIG. 7 schematically shows the guard band provided by the internal computation performed in the DDA.

Essentially the same steps, and very similar structure, are used to achieve subpixel correction in the Depth and other DDA units.

Sample Board-Level Embodiment

A sample board incorporating the GLINT chip may include simply:

the GLINT chip itself, which incorporates a PCI interface;

Video RAM (VRAM), to which the chip has read-write access through its frame buffer (FB) port;

DRAM, which provides a local buffer then made for such purposes as Z buffering; and

a RAMDAC, which provides analog color values in accordance with the color values read out from the VRAM.

Thus one of the advantages of the chip of the presently preferred embodiment is that a minimal board implementation is a trivial task.

FIG. 3A shows a sample graphics board which incorporates the chip of FIG. 2A.

FIG. 3B shows another sample graphics board implementation, which differs from the board of FIG. 3A in that more memory and an additional component is used to achieve higher performance.

FIG. 3C shows another graphics board, in which the chip of FIG. 2A shares access to a common frame store with GUI accelerator chip.

FIG. 3D shows another graphics board, in which the chip of FIG. 2A shares access to a common frame store with a video coprocessor (which may be used for video capture and playback functions (e.g. frame grabbing).

According to a disclosed class of innovative embodiments, there is provided: a graphics system comprising: a rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers; one or more digital differential analyzer units (DDA units) connected to receive said sequence of pixel location identifiers and associated operation specifiers from said rasterizer; wherein ones of said operation specifiers generated by said rasterizer include commands for programming said DDA units to perform incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer include at least one delta value for incrementing, and also include a subpixel offset values which indicate a value proportional to said delta value; and wherein said DDA units generate said incremental changes in pixel attributes, from said delta value and said subpixel offset value, using calculations which use multiplexing operations but do not use any multiply operations.

According to another disclosed class of innovative embodiments, there is provided: a graphics system comprising: a rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers; one or more digital differential analyzer units (DDA units) connected to receive said sequence of pixel location identifiers and associated operation specifiers from said rasterizer; wherein ones of said operation specifiers generated by said rasterizer include commands for programming said DDA units to perform incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values, and wherein said DDA units use said subpixel offset values in calculating pixel attributes, but do not use said subpixel offset values as inputs to any multiply; and wherein said subpixel offset values have a resolution between one and six bits, inclusive.

According to another disclosed class of innovative embodiments, there is provided: a graphics system comprising: a rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers; one or more digital differential analyzer units (DDA units) connected to receive said sequence of pixel location identifiers and associated operation specifiers from said rasterizer; wherein ones of said operation specifiers generated by said rasterizer include commands for programming said DDA units to perform incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values, and wherein said DDA units use said subpixel offset values in calculating pixel attributes, but do not use said subpixel offset values as inputs to any floating-point multiply; and wherein said DDA units calculate subpixel offset values with an internal resolution which includes a range of more than one pixel width, and with a guard band such that calculated subpixel offsets which exceed the width of a pixel do not wrap to zero, but are clamped at a maximum value until the computed value comes down below the maximum.

According to another disclosed class of innovative embodiments, there is provided: a rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers, comprising: means for generating operation specifiers which include commands for programming one or more downstream DDA units to perform incremental changes in pixel attributes; wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values specified as multiplexer inputs; means for outputting said operation specifiers for rendering by units which include one or more DDA units.

According to another disclosed class of innovative embodiments, there is provided: a method for rendering, comprising the steps of: generating a sequence of pixel location identifiers and associated operation specifiers, in a rasterizer; wherein ones of said operation specifiers include commands for incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values; receiving said sequence of pixel location identifiers and associated operation specifiers in one or more digital differential analyzer units (DDA units); and using said subpixel offset values, said DDA units, for calculating pixel attributes, but not as inputs to any multiply operation; and wherein said DDA units calculate subpixel offset values with an internal resolution which includes a range of more than one pixel width, and with a guard band such that calculated subpixel offsets which exceed the width of a pixel do not wrap to zero, but are clamped at a maximum value until the computed value comes down below the maximum.

According to another disclosed class of innovative embodiments, there is provided: a digital differential analyzer unit (DDA unit), comprising: an input connection for receiving sequences of operation specifiers from a rasterizer, individual ones of said operation specifiers including commands for programming said DDA units to generate a series of pixel outputs, in which at least one parameter is incrementally changed over a respective sequence of pixels; internal calculation logic configured to calculate said parameter for each pixel in said respective sequence of pixels, with N bits of resolution; and an output connection connected to output said parameter for each pixel in said respective sequence of pixels, as a digital value with M bits of resolution, wherein M is less than N; wherein said internal calculation logic includes one or more most-significant-bits which are not included in said N bits; whereby, when said internal calculation logic obtains a parameter value which would overflow said M bits of output, said internal calculation logic outputs a M-bit clamped value.

Modifications and Variations

As will be recognized by those skilled in the art, the innovative concepts described in the present application provide a fundamental new approach to computer graphics architectures. Accordingly, a number of pioneering concepts are presented herein. These concepts can be modified and varied over a tremendous range of applications, and accordingly the scope of patented subject matter is not limited by any of the specific exemplary teachings given. For example, as will be obvious to those of ordinary skill in the art, other circuit elements can be added to, or substituted into, the specific circuit topologies shown.

The foregoing text has indicated a large number of alternative implementations, particularly at the higher levels, but these are merely a few examples of the huge range of possible variations.

For example, the preferred chip context can be combined with other functions, or distributed among other chips, as will be apparent to those of ordinary skill in the art.

For another example, the described graphics systems and subsystems can be used, in various adaptations, not only in high-end PC's, but also in workstations, arcade games, and high-end simulators. 

What is claimed is:
 1. A graphics system comprising:a rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers; one or more digital differential analyzer units (DDA units) connected to receive said sequence of pixel location identifiers and associated operation specifiers from said rasterizer; wherein ones of said operation specifiers generated by said rasterizer include commands for programming said DDA units to perform incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer include at least one delta value for incrementing, and also include a subpixel offset values which indicate a value proportional to said delta value; and wherein said DDA units generate said incremental changes in pixel attributes, from said delta value and said subpixel offset value, using calculations which use multiplexing operations but do not use any multiply operations.
 2. The graphics processing subsystem of claim 1, wherein said delta value includes not only a first incrementation value for incrementing in the direction of a scanline, but also a second incrementation value for incrementing along the leading edge of the primitive being rendered.
 3. The graphics processing subsystem of claim 2, wherein said second incrementation value contains at least two non-zero scalar components.
 4. The graphics processing subsystem of claim 1, wherein said rasterizer and all of said DDA units are integrated into a single integrated circuit.
 5. The graphics processing subsystem of claim 1, wherein said rasterizer and said DDA units are interconnected in a pipeline architecture.
 6. The graphics processing subsystem of claim 1, wherein said DDA units include multiple color DDA units and at least one other DDA unit.
 7. The graphics processing subsystem of claim 1, wherein said DDA units include at least one color DDA unit and a depth DDA unit.
 8. A graphics system comprising:a rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers; one or more digital differential analyzer units (DDA units) connected to receive said sequence of pixel location identifiers and associated operation specifiers from said rasterizer; wherein ones of said operation specifiers generated by said rasterizer include commands for programming said DDA units to perform incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values, and wherein said DDA units use said subpixel offset values in calculating pixel attributes, but do not use said subpixel offset values as inputs to any multiply; and wherein said subpixel offset values have a resolution between one and six bits, inclusive.
 9. The graphics processing subsystem of claim 8, wherein said rasterizer and all of said DDA units are integrated into a single integrated circuit.
 10. The graphics processing subsystem of claim 8, wherein said rasterizer and said DDA units are interconnected in a pipeline architecture.
 11. The graphics processing subsystem of claim 8, wherein said DDA units include multiple color DDA units and at least one other DDA unit.
 12. The graphics processing subsystem of claim 8, wherein said DDA units include at least one color DDA unit and a depth DDA unit.
 13. A graphics system comprising:a rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers; one or more digital differential analyzer units (DDA units) connected to receive said sequence of pixel location identifiers and associated operation specifiers from said rasterizer; wherein ones of said operation specifiers generated by said rasterizer include commands for programming said DDA units to perform incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values, and wherein said DDA units use said subpixel offset values in calculating pixel attributes, but do not use said subpixel offset values as inputs to any floating-point multiply; and wherein said DDA units calculate subpixel offset values with an internal resolution which includes a range of more than one pixel width, and with a guard band such that calculated subpixel offsets which exceed the width of a pixel do not wrap to zero, but are clamped at a maximum value until the computed value comes down below the maximum.
 14. The graphics processing subsystem of claim 13, wherein said rasterizer and all of said DDA units are integrated into a single integrated circuit.
 15. The graphics processing subsystem of claim 13, wherein said rasterizer and said DDA units are interconnected in a pipeline architecture.
 16. The graphics processing subsystem of claim 13, wherein said DDA units include multiple color DDA units and at least one other DDA unit.
 17. The graphics processing subsystem of claim 13, wherein said DDA units include at least one color DDA unit and a depth DDA unit.
 18. A rasterizer which generates a sequence of pixel location identifiers and associated operation specifiers, comprising:means for generating operation specifiers which include commands for programming one or more downstream DDA units to perform incremental changes in pixel attributes; wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values specified as multiplexer inputs; means for outputting said operation specifiers for rendering by units which include one or more DDA units.
 19. A method for rendering, comprising the steps of:generating a sequence of pixel location identifiers and associated operation specifiers, in a rasterizer; wherein ones of said operation specifiers include commands for incremental changes in pixel attributes; and wherein ones of said operation specifiers generated by said rasterizer also include subpixel offset values; receiving said sequence of pixel location identifiers and associated operation specifiers in one or more digital differential analyzer units (DDA units); and using said subpixel offset values, said DDA units, for calculating pixel attributes, but not as inputs to any multiply operation; and wherein said DDA units calculate subpixel offset values with an internal resolution which includes a range of more than one pixel width, and with a guard band such that calculated subpixel offsets which exceed the width of a pixel do not wrap to zero, but are clamped at a maximum value until the computed value comes down below the maximum.
 20. A digital differential analyzer unit (DDA unit), comprising:an input connection for receiving sequences of operation specifiers from a rasterizer, individual ones of said operation specifiers including commands for programming said DDA units to generate a series of pixel outputs, in which at least one parameter is incrementally changed over a respective sequence of pixels; internal calculation logic configured to calculate said parameter for each pixel in said respective sequence of pixels, with N bits of resolution; and an output connection connected to output said parameter for each pixel in said respective sequence of pixels, as a digital value with M bits of resolution, wherein M is less than N; wherein said internal calculation logic includes one or more most-significant-bits which are not included in said N bits; whereby, when said internal calculation logic obtains a parameter value which would overflow said M bits of output, said internal calculation logic outputs a M-bit clamped value.
 21. The DDA unit of claim 20, wherein M is less than N-1.
 22. The DDA unit of claim 20, wherein M is less than N-3.
 23. The DDA unit of claim 20, wherein said parameter is a color value.
 24. The DDA unit of claim 20, wherein said parameter is a depth value.
 25. The DDA unit of claim 20, wherein said DDA unit is included within a single block of a pipelined multiprocessor architecture. 